Fix TestMergeIntoTable2 test

For the test TestMergeIntoTable2@"Test only insert for source table in dup key without preCombineField", after adding "
spark.sql(s"set ${MERGE_SMALL_FILE_GROUP_CANDIDATES_LIMIT.key} = 0")", the test fails:
{code:java}
Expected Array([1,a2,10.4,1004,2021-03-21], [1,a2,10.4,1004,2021-03-21], [3,a3,10.3,1003,2021-03-21]), but got Array([1,a2,10.2,1002,2021-03-21], [1,a2,10.4,1004,2021-03-21], [3,a3,10.3,1003,2021-03-21])
ScalaTestFailureLocation: org.apache.spark.sql.hudi.HoodieSparkSqlTestBase at (HoodieSparkSqlTestBase.scala:109)
org.scalatest.exceptions.TestFailedException: Expected Array([1,a2,10.4,1004,2021-03-21], [1,a2,10.4,1004,2021-03-21], [3,a3,10.3,1003,2021-03-21]), but got Array([1,a2,10.2,1002,2021-03-21], [1,a2,10.4,1004,2021-03-21], [3,a3,10.3,1003,2021-03-21])
    at org.scalatest.Assertions.newAssertionFailedException(Assertions.scala:472)
    at org.scalatest.Assertions.newAssertionFailedException$(Assertions.scala:471)
    at org.scalatest.funsuite.AnyFunSuite.newAssertionFailedException(AnyFunSuite.scala:1562)
    at org.scalatest.Assertions.assertResult(Assertions.scala:867)
    at org.scalatest.Assertions.assertResult$(Assertions.scala:863)
    at org.scalatest.funsuite.AnyFunSuite.assertResult(AnyFunSuite.scala:1562)
    at org.apache.spark.sql.hudi.HoodieSparkSqlTestBase.checkAnswer(HoodieSparkSqlTestBase.scala:109)
    at org.apache.spark.sql.hudi.TestMergeIntoTable2.$anonfun$new$36(TestMergeIntoTable2.scala:897)
    at org.apache.spark.sql.hudi.TestMergeIntoTable2.$anonfun$new$36$adapted(TestMergeIntoTable2.scala:841)
    at org.apache.spark.sql.hudi.HoodieSparkSqlTestBase.withTempDir(HoodieSparkSqlTestBase.scala:77)
    at org.apache.spark.sql.hudi.TestMergeIntoTable2.$anonfun$new$35(TestMergeIntoTable2.scala:841)
    at org.apache.spark.sql.hudi.TestMergeIntoTable2.$anonfun$new$35$adapted(TestMergeIntoTable2.scala:840)
    at scala.collection.immutable.List.foreach(List.scala:392)
    at org.apache.spark.sql.hudi.TestMergeIntoTable2.$anonfun$new$34(TestMergeIntoTable2.scala:840)
    at scala.runtime.java8.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:23)
    at org.apache.spark.sql.hudi.HoodieSparkSqlTestBase.$anonfun$test$1(HoodieSparkSqlTestBase.scala:85)
    at org.scalatest.OutcomeOf.outcomeOf(OutcomeOf.scala:85)
    at org.scalatest.OutcomeOf.outcomeOf$(OutcomeOf.scala:83)
    at org.scalatest.OutcomeOf$.outcomeOf(OutcomeOf.scala:104)
    at org.scalatest.Transformer.apply(Transformer.scala:22)
    at org.scalatest.Transformer.apply(Transformer.scala:20)
    at org.scalatest.funsuite.AnyFunSuiteLike$$anon$1.apply(AnyFunSuiteLike.scala:189)
    at org.scalatest.TestSuite.withFixture(TestSuite.scala:196)
    at org.scalatest.TestSuite.withFixture$(TestSuite.scala:195)
    at org.scalatest.funsuite.AnyFunSuite.withFixture(AnyFunSuite.scala:1562)
    at org.scalatest.funsuite.AnyFunSuiteLike.invokeWithFixture$1(AnyFunSuiteLike.scala:187)
    at org.scalatest.funsuite.AnyFunSuiteLike.$anonfun$runTest$1(AnyFunSuiteLike.scala:199)
    at org.scalatest.SuperEngine.runTestImpl(Engine.scala:306)
    at org.scalatest.funsuite.AnyFunSuiteLike.runTest(AnyFunSuiteLike.scala:199)
    at org.scalatest.funsuite.AnyFunSuiteLike.runTest$(AnyFunSuiteLike.scala:181)
    at org.scalatest.funsuite.AnyFunSuite.runTest(AnyFunSuite.scala:1562)
    at org.scalatest.funsuite.AnyFunSuiteLike.$anonfun$runTests$1(AnyFunSuiteLike.scala:232)
    at org.scalatest.SuperEngine.$anonfun$runTestsInBranch$1(Engine.scala:413)
    at scala.collection.immutable.List.foreach(List.scala:392)
    at org.scalatest.SuperEngine.traverseSubNodes$1(Engine.scala:401)
    at org.scalatest.SuperEngine.runTestsInBranch(Engine.scala:396)
    at org.scalatest.SuperEngine.runTestsImpl(Engine.scala:475)
    at org.scalatest.funsuite.AnyFunSuiteLike.runTests(AnyFunSuiteLike.scala:232)
    at org.scalatest.funsuite.AnyFunSuiteLike.runTests$(AnyFunSuiteLike.scala:231)
    at org.scalatest.funsuite.AnyFunSuite.runTests(AnyFunSuite.scala:1562)
    at org.scalatest.Suite.run(Suite.scala:1112)
    at org.scalatest.Suite.run$(Suite.scala:1094)
    at org.scalatest.funsuite.AnyFunSuite.org$scalatest$funsuite$AnyFunSuiteLike$$super$run(AnyFunSuite.scala:1562)
    at org.scalatest.funsuite.AnyFunSuiteLike.$anonfun$run$1(AnyFunSuiteLike.scala:236)
    at org.scalatest.SuperEngine.runImpl(Engine.scala:535)
    at org.scalatest.funsuite.AnyFunSuiteLike.run(AnyFunSuiteLike.scala:236)
    at org.scalatest.funsuite.AnyFunSuiteLike.run$(AnyFunSuiteLike.scala:235)
    at org.apache.spark.sql.hudi.HoodieSparkSqlTestBase.org$scalatest$BeforeAndAfterAll$$super$run(HoodieSparkSqlTestBase.scala:44)
    at org.scalatest.BeforeAndAfterAll.liftedTree1$1(BeforeAndAfterAll.scala:213)
    at org.scalatest.BeforeAndAfterAll.run(BeforeAndAfterAll.scala:210)
    at org.scalatest.BeforeAndAfterAll.run$(BeforeAndAfterAll.scala:208)
    at org.apache.spark.sql.hudi.HoodieSparkSqlTestBase.run(HoodieSparkSqlTestBase.scala:44)
    at org.scalatest.tools.SuiteRunner.run(SuiteRunner.scala:45)
    at org.scalatest.tools.Runner$.$anonfun$doRunRunRunDaDoRunRun$13(Runner.scala:1314)
    at org.scalatest.tools.Runner$.$anonfun$doRunRunRunDaDoRunRun$13$adapted(Runner.scala:1308)
    at scala.collection.immutable.List.foreach(List.scala:392)
    at org.scalatest.tools.Runner$.doRunRunRunDaDoRunRun(Runner.scala:1308)
    at org.scalatest.tools.Runner$.$anonfun$runOptionallyWithPassFailReporter$24(Runner.scala:993)
    at org.scalatest.tools.Runner$.$anonfun$runOptionallyWithPassFailReporter$24$adapted(Runner.scala:971)
    at org.scalatest.tools.Runner$.withClassLoaderAndDispatchReporter(Runner.scala:1474)
    at org.scalatest.tools.Runner$.runOptionallyWithPassFailReporter(Runner.scala:971)
    at org.scalatest.tools.Runner$.run(Runner.scala:798)
    at org.scalatest.tools.Runner.run(Runner.scala)
    at org.jetbrains.plugins.scala.testingSupport.scalaTest.ScalaTestRunner.runScalaTest2or3(ScalaTestRunner.java:43)
    at org.jetbrains.plugins.scala.testingSupport.scalaTest.ScalaTestRunner.main(ScalaTestRunner.java:26) {code}

## JIRA info

- Link: https://issues.apache.org/jira/browse/HUDI-6978
- Type: Improvement

Start in TestMergeIntoTable2.scala around lines 841-897, focusing on the TestMergeIntoTable2 case named "Test only insert for source table in dup key without preCombineField" and its MERGE_SMALL_FILE_GROUP_CANDIDATES_LIMIT setting. Run the test and inspect the checkAnswer failure in HoodieSparkSqlTestBase; done means the test produces the intended result consistently after that setting is added.

Written by the indexing model from the issue text.