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This change moves all test-related setup and teardown to a separate test
environment. It also fixes a small bug that causes a crash when destroying the
video player without playing a video.

TEST=ran new VD tests on nocturne

BUG=879065

Change-Id: I0f746ff3aff1598130d563aa8241e87f3d739c8a
Reviewed-on: https://chromium-review.googlesource.com/c/1350413Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: David Staessens <dstaessens@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612083}