Prevent improper use of testing framework APIs

Prior to this commit, a lot of work had been done to prevent improper
use of testing Framework APIs throughout the codebase; however, there
were still some loopholes.

This commit addresses these loopholes by introducing additional
Checkstyle rules (and modifying existing rules) to prevent improper use
of testing framework APIs in production code as well as in test code.

- Checkstyle rules for banned imports have been refactored into
  multiple rules specific to JUnit 3, JUnit 4, JUnit Jupiter, and
  TestNG.
- Accidental usage of org.junit.Assume has been switched to
  org.junit.jupiter.api.Assumptions.
- All test classes now reside under org.springframework packages.
- All test classes (including abstract test classes) now conform to the
  `*Tests` naming convention.
  - As an added bonus, tests in the renamed
    ScenariosForSpringSecurityExpressionTests are now included in the
    build.
- Dead JUnit 4 parameterized code has been removed from
  DefaultServerWebExchangeCheckNotModifiedTests.

Closes gh-22962

spring-context/src/test/java/org/springframework/jmx/access/MBeanClientInterceptorTests.java

@@ -42,7 +42,7 @@ import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 import static org.assertj.core.api.Assertions.assertThatIOException;
 import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
-import static org.junit.Assume.assumeTrue;
+import static org.junit.jupiter.api.Assumptions.assumeTrue;
 
 /**
  * @author Rob Harrop