[libc++][NFC] Fix copy-paste errors in tests

The test precision_type.pass.cpp was a duplicate of precision.pass.cpp,
so it is removed. atomic_flag_test.pass.cpp was a duplicate of
atomic_flag_test_and_set.pass.cpp, so instead I wrote a proper
test for it. Those duplicate tests were detected with

     find libcxx ! -empty -type f -exec md5sum {} + | sort | uniq -w32 -dD