Add module to support testing of generated code

Add a new unpublished `spring-core-test` module to support testing of
generated code. The module include a `TestCompiler` class which can be
used to dynamically compile generated Java code. It also include an
AssertJ friendly `SourceFile` class which uses qdox to provide targeted
assertions on specific parts of a generated source file.

See gh-28120

framework-bom/framework-bom.gradle

@@ -7,7 +7,7 @@ group = "org.springframework"
 
 dependencies {
 	constraints {
-		parent.moduleProjects.sort { "$it.name" }.each {
+		parent.moduleProjects.findAll{ it.name != 'spring-core-test' }.sort{ "$it.name" }.each {
 			api it
 		}
 	}