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