Migrate tests to JUnit Jupiter

[fmbenhassine]

This issue is to track the migration of the test suite from JUnit 4 to JUnit 5:

• spring-batch-infrastructure: Migrate Spring Batch Infrastructure to JUnit Jupiter #4125
• spring-batch-core: Migrate Spring Batch Core to JUnit Jupiter #4172
• spring-batch-test: Migrate Spring Batch Test to JUnit Jupiter #4174
• spring-batch-integration: Migrate Spring Batch Integration to JUnit Jupiter #4124
• spring-batch-samples: Migrate Spring Batch Samples to JUnit Jupiter #4173

[fmbenhassine]

Refinements like those done for spring-batch-infrastructure in #4175 can be done later for other modules. Improving test suites is a non-ending task, so I'm closing this for now as it is already a huge improvement to Spring Batch's test suite.

A BIG thank you to @hpoettker for this absolutely amazing contribution! REALLY appreciated 👍