Clean up scm_unittest.py to not rely on static methods.

This changes all of the test scaffolding to use instance methods on the
TestCase objects and use Executive objects properly instead of talking directly
to the underlying Python APIs.

This patch is just cleanup; it does not make the tests execute faster or
anything like that.

R=ojan@chromium.org, eseidel@chromium.org, alancutter@chromium.org
BUG=

Review URL: https://codereview.chromium.org/27022006

git-svn-id: svn://svn.chromium.org/blink/trunk@159618 bbb929c8-8fbe-4397-9dbb-9b2b20218538