Fix refs to with_system_account decorator

**This should not be merged until the corresponding pull request described below has been merged. These tests will fail, and the issue should be closed and re-opened after the corresponding pull request has been merged, in order to re-test with the new salt-testing code.**

This renames this decorator to with_system_user, to go along with
changes in https://github.com/saltstack/salt-testing/pull/14.