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Add a new unpublished `spring-core-test` module to support testing of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

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

@@ -0,0 +1,13 @@
+import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
+
+description = "Spring Core Test"
+
+dependencies {
+	api(project(":spring-core"))
+	api("org.assertj:assertj-core")
+	api("com.thoughtworks.qdox:qdox")
+}
+
+tasks.withType(PublishToMavenRepository).configureEach {
+	it.enabled = false
+}