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

spring-core-test/spring-core-test.gradle

@@ -4,6 +4,9 @@ dependencies {
 	api(project(":spring-core"))
 	api("org.assertj:assertj-core")
 	api("com.thoughtworks.qdox:qdox")
+	compileOnly("org.junit.jupiter:junit-jupiter")
+	compileOnly("org.junit.platform:junit-platform-engine")
+	compileOnly("org.junit.platform:junit-platform-launcher")
 }
 
 tasks.withType(PublishToMavenRepository).configureEach {