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The CL adds test coverage for CrOSComponentInstaller::Load method.
The tests set up fake components in a scoped test dir (containing
only manifest files) which is used to override component paths provided
by path service, and verify that Load requests load appropriate
component, issue on demand update requests as expected, and issue
requests to load the components to image loader service.

The focus in this cl is to add tests for behavior used for loading
demo-mode-resources component, but the set of tests can be expanded in
the future.

Change-Id: Ib4c2fab67492f1e62a3a1b3f1343139b9aa5baeb
Reviewed-on: https://chromium-review.googlesource.com/c/1278635Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Xiaochu Liu <xiaochu@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601214}