[test] Split strip-preserve-time.test, and skip atime test on NetBSD

Split timestamp preservation tests into atime and mtime test, and skip
the former on NetBSD.  When the filesystem is mounted noatime, NetBSD
not only inhibits implicit atime updates but also prevents setting atime
via utime(), causing the test to fail.

Differential Revision: https://reviews.llvm.org/D55271

llvm-svn: 348354