Restructure test suite to follow libc++ standard layout
Summary: Subsumes changes requested in https://reviews.llvm.org/D59110
Reviewers: EricWF, ldionne
Subscribers: mgorny, krytarowski, jfb, jdoerfert, libcxx-commits
Differential Revision: https://reviews.llvm.org/D59856
llvm-svn: 357124