DeleteManagerTest failure

There seems to be an intermittent failure in `DeleteManagerTest`
```
com.github.ambry.router.DeleteManagerTest > testBadCallback FAILED
java.lang.AssertionError: All operations should be finished. expected:<0> but was:<1>
at org.junit.Assert.fail(Assert.java:91)
at org.junit.Assert.failNotEquals(Assert.java:645)
at org.junit.Assert.assertEquals(Assert.java:126)
at org.junit.Assert.assertEquals(Assert.java:470)
at com.github.ambry.router.DeleteManagerTest$2.testAndAssert(DeleteManagerTest.java:161)
at com.github.ambry.router.RouterTestHelpers.testWithErrorCodes(RouterTestHelpers.java:79)
at com.github.ambry.router.RouterTestHelpers.testWithErrorCodes(RouterTestHelpers.java:62)
at com.github.ambry.router.DeleteManagerTest.testBadCallback(DeleteManagerTest.java:133)
```
Have the background deleter operations been accounted for in these tests? @cgtz @xiahome @pnarayanan

Start by running DeleteManagerTest.testBadCallback and read the failing assertion at DeleteManagerTest.java:161, along with RouterTestHelpers.java:62 and :79. Investigate whether background deleter operations remain active when the test checks completion; done means the intermittent failure is resolved and all operations are accounted for.

Written by the indexing model from the issue text.