Fix a minor test bug in test-webkitpy

Fix a minor test bug in test-webkitpy

main_unittest was relying on the original sys.argv and passing that
to the arg-parsing code. If somehow you had something in argv that
wasn't a valid argument to test-webkitpy (e.g., by testing w/
unittest directly), the test would fail and raise SystemExit.

This was an unnecessarily fragile design, so I fixed to preserve
the original sys.argv and replace it for the test w/ something predictable.

R=eseidel@chromium.org
BUG=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=178484

[eseidel, 2014-07-18 18:52:28]

lgtm

[commit-bot: I haz the power, 2014-07-18 18:52:49]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/dpranke@chromium.org/397723004/1

[commit-bot: I haz the power, 2014-07-18 20:10:50]

Change committed as 178484