Add mock_crostini rule for FilesApp foreground unittests

 - add background BUILD rule for mock_crostini
   - make mock_crostini visible to foreground code
 - include mock code in foreground unittests.html
 - change background / foreground tests to use it

No change in test behavior.

Bug: 911024
Change-Id: I15e3b243cea5937111da66ac504e143116bcbf0d
Reviewed-on: https://chromium-review.googlesource.com/c/1358321
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613415}
7 files changed