Add `SpringJUnit5` checkstyle rule to ensure that JUnit 4 annotations
aren't accidentally used in new tests.

The "must not be public" rule has been suppressed since there are
quite a few tests that extend base tests from other packages.

Closes gh-23181

This commit migrates to the MockitoJUnitRunner where sensible, which
will later allow for an easier migration to Mockito's extension for
JUnit Jupiter.

In addition, this commit deletes unnecessary stubbing for various mocks
and polishes test fixture setup in various test classes.

Migrate all existing TestNG based assertions to AssertJ and add
Checkstyle rules to ensure they don't return.

See gh-23022

Migrate all existing JUnit Jupiter based assertions to AssertJ and add
Checkstyle rules to ensure they don't return.

See gh-23022

Migrate all existing JUnit 4 `assert...` based assertions to AssertJ
and add a checkstyle rule to ensure they don't return.

See gh-23022

Migrate all existing `assertThat(..., Matcher)` assertions to AssertJ
and add checkstyle rules to ensure they don't return.

See gh-23022

Migrate tests that use `@test(expectedException=...)` or
`try...fail...catch` to use AssertJ's `assertThatException`
instead.

This commit configures Checkstyle to enforces static imports for JUnit 4
assertions and assumptions.

See gh-22932

This commit adds Checkstyle rules to disallow imports of the following.

- junit.framework.*
- org.junit.Assert.assertThat
- org.junit.Assume.assumeThat

See gh-22894

Migrate all tests to consistently use BDD Mockito. Also add
checksyle rule to enforce going forwards.

* Add limited checkstyles to test code

Add a limited set of checkstyle rules to the test codebase to improve
code consistency.

* Fix checksyle violations in test code

* Organize imports to fix checkstyle for test code

* Migrate to assertThatExceptionOfType

Migrate aware from ExpectedException rules to AssertJ exception
assertions. Also include a checkstyle rules to ensure that the
the ExpectedException is not accidentally used in the future.

See gh-22894

See gh-22634

See http://checkstyle.sourceforge.net/config.html#Configuration_XML_Structure

Add checkstyle to the build to enforce Spring Framework coding
conventions. Rule suppressions are used for several classes where
relaxing the rules makes more sense than fixing them.

Issue: SPR-16968