Need Junit 5 Support

I was trying to use spark-testing-base with Junit 5 but even after extending SharedJavaSparkContext I was getting jsc() as null. On a quick look I found that SharedJavaSparkContext is expecting junit 4(org.junit.Test).So in my method with junit5 @Test(org.junit.jupiter.api.Test) jsc() was coming as null. If I move to junit4 everything works fine.As we are moving to junit5 I believe this is a must have change in the lib.

Start by reading the SharedJavaSparkContext entry point and tracing how jsc() is initialized from JUnit 4 annotations. Compare that behavior with JUnit 5 annotations and identify the tests covering the shared Spark context. Done means a JUnit 5 test extending SharedJavaSparkContext receives a non-null jsc() while existing JUnit 4 behavior continues to work.

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