Get output from hung mojo apptests.
Depends on https://codereview.chromium.org/
1239563003
Start Xvfb and Openbox separately from the test process.
This lets us get output from killed hung test processes.
(wrapping the commandline with xvfb.py prevents this)
Remove xvfb from Paths objects. Use more single quotes.
TODO: Use test_env.run_executable() or its env vars?
BUG=493535,504917
TEST=Apptests killed for timeouts yield output.
R=maruel@chromium.org
TBR=ben@chromium.org,sky@chromium.org
Review URL: https://codereview.chromium.org/
1230683002
Cr-Commit-Position: refs/heads/master@{#338943}