test: Make it possible to opt in to use_clang_verify per test

This modifies the use_clang_verify parameter I added in r217009 to
only apply to tests that specifically ask for it via // USE_VERIFY.
This allows us to incrementally convert tests, but start enjoying the
benefits right away.

Suggested by Eric Fiselier in code review.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@217017 91177308-0d34-0410-b5e6-96231b3b80d8