Prior to this commit, the TestRuntimeHintsRegistrar API combined
processing of MergedContextConfiguration and test classes. However, it
appears that only spring-test internals have a need for registering
hints based on the MergedContextConfiguration. For example, Spring
Boot's AOT testing support has not had such a need, and it is assumed
that third parties likely will not have such a need.
In light of that, this commit simplifies the TestRuntimeHintsRegistrar
API so that it focuses on processing of a single test class.
In addition, this commit moves the hint registration code specific to
MergedContextConfiguration to an internal mechanism.
Closes gh-29264
935265048a