Add SpringJUnit5 checkstyle rule

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.

src/checkstyle/checkstyle-suppressions.xml

@@ -4,6 +4,7 @@
 
 	<!-- global -->
 	<suppress files="[\\/]src[\\/]test[\\/]java[\\/]" checks="AnnotationLocation|AnnotationUseStyle|AtclauseOrder|AvoidNestedBlocks|FinalClass|HideUtilityClassConstructor|InnerTypeLast|JavadocStyle|JavadocType|JavadocVariable|LeftCurly|MultipleVariableDeclarations|NeedBraces|OneTopLevelClass|OuterTypeFilename|RequireThis|SpringCatch|SpringJavadoc|SpringNoThis" />
+	<suppress files="[\\/]src[\\/]test[\\/]java[\\/]" checks="SpringJUnit5" message="should not be public" />
 
 	<!-- spring-beans -->
 	<suppress files="TypeMismatchException" checks="MutableException"/>
@@ -70,6 +71,8 @@
 	<suppress files="HeaderAssertions" checks="IllegalImport" id="bannedHamcrestImports" />
 	<suppress files="StatusAssertions" checks="IllegalImport" id="bannedHamcrestImports" />
 	<suppress files="JsonPathResultMatchers" checks="IllegalImport" id="bannedHamcrestImports" />
+	<suppress files="src[\\/]test[\\/]java[\\/]org[\\/]springframework[\\/]test[\\/]context[\\/]junit4" checks="SpringJUnit5"/>
+	<suppress files="ContextHierarchyDirtiesContextTests|ClassLevelDirtiesContextTests|ContextConfigurationWithPropertiesExtendingPropertiesAndInheritedLoaderTests|ContextConfigurationWithPropertiesExtendingPropertiesTests|DirtiesContextInterfaceTests|.*WacTests|JUnit4SpringContextWebTests"  checks="SpringJUnit5"/>
 
 	<!-- spring-web -->
 	<suppress files="SpringHandlerInstantiator" checks="JavadocStyle" />

src/checkstyle/checkstyle.xml

@@ -211,5 +211,6 @@
 		</module>
 		<module name="io.spring.javaformat.checkstyle.check.SpringCatchCheck" />
 		<module name="io.spring.javaformat.checkstyle.check.SpringJavadocCheck" />
+		<module name="io.spring.javaformat.checkstyle.check.SpringJUnit5Check" />
  	</module>
 </module>