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Historically //ash has been inconsistent in its placement of FooTestApi,
TestFooDelegate, MockFoo, etc.

Top-level ash OWNERS have decided that this sort of test code should
live in the same directory as Foo, not in //ash/test.

Move the files, collapse //ash/test/BUILD.gn into //ash/BUILD.gn and
add a note to //ash/README.md

TBR=reveman@chromium.org

Bug: none
Test: compiles, ash_unittests
Change-Id: I22b556742c37f9a414824d2ce48f3fa261949b71
Reviewed-on: https://chromium-review.googlesource.com/576142
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487559}