Fix two test failures in <experimental/filesystem>

First, <experimental/filesystem> didn't correctly guard
against min/max macros. This adds the proper push/pop macro guards.

Second, an internal time helper had been renamed but the test for
it hadn't been updated. This patch updates those tests.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@337520 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed