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-orm/src/test/java/org/springframework/orm/jpa/support/PersistenceAnnotationBeanPostProcessorTests.java

@@ -32,6 +32,7 @@ import org.springframework.aot.generator.DefaultGeneratedTypeContext;
 import org.springframework.aot.generator.GeneratedType;
 import org.springframework.aot.hint.RuntimeHints;
 import org.springframework.aot.hint.TypeReference;
+import org.springframework.aot.test.generator.compile.CompileWithTargetClassAccess;
 import org.springframework.aot.test.generator.compile.TestCompiler;
 import org.springframework.aot.test.generator.file.SourceFile;
 import org.springframework.aot.test.generator.file.SourceFiles;
@@ -126,6 +127,7 @@ class PersistenceAnnotationBeanPostProcessorTests {
 	}
 
 	@Test
+	@CompileWithTargetClassAccess(classes = DefaultPersistenceUnitField.class)
 	void generateEntityManagerFactoryInjection() {
 		GenericApplicationContext context = new AnnotationConfigApplicationContext();
 		context.registerBeanDefinition("test", new RootBeanDefinition(DefaultPersistenceUnitField.class));