Some filesystems track atime always. This relaxes the test to accept either a
filesystem which does not accurately track atime or does track the atime
accurately. This allows the test to pass on filesystems mounted with
`strictatime` on Linux or on macOS.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@278357 91177308-0d34-0410-b5e6-96231b3b80d8