Spring Introduce TestRuntimeHintsRegistrar in the TestContext framework

Although org.springframework.aot.hint.RuntimeHintsRegistrar can be implemented to register general hints that should be applied for all testing infrastructure, certain use cases require access to test classes and the MergedContextConfiguration in order to register appropriate hints.

In light of that, we need to introduce TestRuntimeHintsRegistrar as a companion to RuntimeHintsRegistrar.