[ASTImporter] Split out Objective-C related unit tests

This moves the two tests we have for importing Objective-C nodes to their own
file. The motivation is that this means I can add more Objective-C tests without
making the compilation time of ASTImporterTest even longer. Also it seems nice
to separate the Apple-specific stuff from the ASTImporter test.

Reviewed By: martong

Differential Revision: https://reviews.llvm.org/D99162