Fix tests for membership to use 'not in'

flake8 reports:

  E713 test for membership should be 'not in'

Change-Id: I4446be67c431b7267105b53478d2ceba2af758d7
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254451


Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>