This tests some high-level scenarios of the V8 test runner, with
fake executable, test-suite extension and build configs.

The runners are slightly modified to be easier testable. Args
are passed from the tests now and the V8 root folder can be faked
by the tests.

We support coverage if python coverage 4.0 is installed. Otherwise
we run without it.

Bug: v8:6917
Change-Id: Ib149fd88027cbdc3382bcaea2d82020582f79d2d
Reviewed-on: https://chromium-review.googlesource.com/831506
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50310}