[autotest] Use sys.exit() instead of exit() in test_importer

See http://stackoverflow.com/questions/6501121/the-difference-between-exit-and-sys-exit-in-python

TEST=None
BUG=None

Change-Id: I4918c8ed01d3c1a3a01b4d7f3a787cf07fa65be0
Reviewed-on: https://gerrit.chromium.org/gerrit/65594
Tested-by: Alex Miller <milleral@chromium.org>
Reviewed-by: Keyar Hood <keyar@chromium.org>
Commit-Queue: Alex Miller <milleral@chromium.org>
1 file changed