Comment parsing tests: move tests where they belong

comment-misc-tags.m is mostly about miscellaneous Doxygen tags.  Move out tests
that check if the comment is attached to an ObjC decl.  Because the exitsting
test for this is in C++ (annotate-comments.cpp), create a new test --
annotate-comments-objc.m.

The rest of comment-misc-tags.m should be also moved elsewhere.