osutils: add Touch unittests
osutils: add Touch unittests

I was thinking about adding other functionality so I started some
unittests, but then I ended up going a different route.  These tests
are still useful though.

BUG=None
TEST=`./osutils_unittest.py` passes

Change-Id: I90c8939e3806a0b50cfb4e88604353574e876003
Reviewed-on: https://gerrit.chromium.org/gerrit/66229
Reviewed-by: Matt Tennant <46e612415b446ccde19638e4035892001c237942@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite