Make JUnit4 dependency optional in spring-batch-test

[murdos]

Expected Behavior

spring-batch-test should not have dependency to JUnit4, or should have an optional dependency to JUnit4.

Current Behavior

spring-batch-test has a compile dependency to JUnit4.

Context

In a project using only JUnit5, JUnit4 is in the test classpath when using spring-batch-test.

[danilopiazza]

Hi, I opened pull request #4054 to address this issue.