[SCM tests] Don't forget to change back to original cwd.

After removing the temporary SCM directory change back to the
original cwd. This will avoid errors on future os.cwd() calls.

BUG=412774
TEST=test-webkitpy --all

Review URL: https://codereview.chromium.org/558103003

git-svn-id: svn://svn.chromium.org/blink/trunk@181894 bbb929c8-8fbe-4397-9dbb-9b2b20218538