Fix FilesystemCopierAction unit tests.
Fix FilesystemCopierAction unit tests.

This will add an explicit failure if either the read source/target
content fails to compare against the original. This way we can be sure
whichever has caused a failure when one is exhibited.

BUG=chromium-os:29841
TEST=Builds and passes unit tests

Change-Id: I685ddf9a383c0bddfea22369a128c7d8c56b0295
Reviewed-on: https://gerrit.chromium.org/gerrit/25127
Reviewed-by: Don Garrett <546ec366615570e68497e3c5df9092106a9808f4@chromium.org>
Commit-Ready: Gilad Arnold <a0dab390cfdc583218cee97b60c5c4eb1be355d2@chromium.org>
Tested-by: Gilad Arnold <a0dab390cfdc583218cee97b60c5c4eb1be355d2@chromium.org>

mjg59/update_engine,endocode/update_engine,marineam/update_engine,mjg59/update_engine,endocode/update_engine,marineam/update_engine,marineam/update_engine,marineam/update_engine,mjg59/update_engine,endocode/update_engine