Filesystem tests: un-confuse write time

Summary:
The filesystem test was confused about access versus write / modification time. The spec says:

  file_time_type last_write_time(const path& p, error_code& ec) noexcept;
  Returns: The time of last data modification of p, determined as if by the value of the POSIX stat structure member st_mtime obtained as if by POSIX stat(). The signature with argument ec returns file_time_type::min() if an error occurs.

The test was looking at st_atime, not st_mtime, when comparing the result from last_write_time. That was probably due to using a pair instead of naming things nicely or using types. I opted to rename things so it's clearer.

This used to cause test bot failures.

<rdar://problem/40648859>

Reviewers: EricWF, mclow.lists, aemerson

Subscribers: christof, cfe-commits

Differential Revision: https://reviews.llvm.org/D47557

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@333723 91177308-0d34-0410-b5e6-96231b3b80d8