Polish Spring Batch Infrastructure tests

[hpoettker]

This PR is in scope of #4166 polishes the tests in Spring Batch Infrastructure that have been migrated to JUnit 5 in #4125.

The goal of the PR is to improve the readability of the tests by reducing visual clutter and using more concise test constructs where directly available, i.e.

• test classes and methods are now package-private where possible
• not-required throws clauses are removed from test method signatures
• all Assertions are changed to static imports
• assertThrows and assertDoesNotThrow are used where previoulsy try, catch, and fail were used in a much more verbose style
• @MockitoSettings is used to set strictness of mocking at the class level

Some final modifiers have been added to class variables to make clearer whether they are expected to change or not during the tests, respectively.

[hpoettker]

I rebased on main to fix the merge conflict with #4171. RecordFieldExtractorTests is no longer changed in this PR.

[fmbenhassine]

I rebased on main to fix the merge conflict with #4171. RecordFieldExtractorTests is no longer changed in this PR.

Great! I was about to do it locally. Thank you for the update!