Fix test from r214577 for other timezones

Unsurprisingly, changing a file modification time to a specific
date/time doesn't give the same epoch time everywhere. Just make the
file move into the past and look at only the first few digits of the
epoch time.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214589 91177308-0d34-0410-b5e6-96231b3b80d8