Make uitests more robust

  * When fetching trees from the database the queries are now scoped to
    the test instance to prevent other data in the database from
    interfering with assertions.

  * I was running into intermittent failures in some of the tests which,
    showed up more when running with a virtual display. Adding delays
    between steps appeared to resolve the issue.
Merge pull request #840 from jwalgran/fix/uitest

Make uitests more robust