Improve TestCompiler and allow lookup based class defines

Update the `TestCompiler` so that classes can be defined using
a `Lookup`. This update allows package-private classes to be
accessed without needing a quite so unusual classloader setup.

The `@CompileWithTargetClassAccess` should be added to any
test that needs to use `Lookup` based defines. The test will
run with a completed forked classloader so not to pollute the
main classloader.

This commit also adds some useful additional APIs.

See gh-28120

src/checkstyle/checkstyle-suppressions.xml

@@ -38,6 +38,9 @@
 	<suppress files="ResolvableType" checks="FinalClass"/>
 	<suppress files="[\\/]src[\\/]testFixtures[\\/]java[\\/].+" checks="IllegalImport" id="bannedJUnitJupiterImports"/>
 
+	<!-- spring-core-test -->
+	<suppress files="CompileWithTargetClassAccess" checks="IllegalImport" id="bannedJUnitJupiterImports" />
+
 	<!-- spring-expression -->
 	<suppress files="ExpressionException" checks="MutableException"/>
 	<suppress files="SpelMessage" checks="JavadocVariable|JavadocStyle"/>