Add skip check to tests that modify the user source

When using an immutable user source, test should skip if the
test tries to modify the user source. This includes creating,
updating and deleting users. A similar change was merged here:
https://review.opendev.org/#/c/670590/

Change-Id: If7c6ae7fc57a4ac256cf668c4075ee86143202ea
5 files changed