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=

Review URL: https://codereview.chromium.org/397723004

git-svn-id: svn://svn.chromium.org/blink/trunk@178484 bbb929c8-8fbe-4397-9dbb-9b2b20218538