Migration Flink from Junit4 to Junit5

Based on the consensus from the mailing list discussion[1][2], we have been starting working on the JUnit4 to JUnit5 migration. 

This is the umbrella ticket which describes the big picture of the migration with following steps:

• AssertJ integration and guideline
• Test Framework upgrade from JUnit4 to JUnit5
• JUnit5 migration guideline(document and reference migration)
• Optimization for issues found while writing new test in JUint5
• [Long-term]Module based graceful migration of old tests in JUnit4 to JUnit5

[1]

[DISCUSS]Moving to JUnit5

[2] [DISCUSS] Conventions on assertions to use in tests