Some tests fail when run as root

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/863)
In GitLab by [[Gitlab user @cs-shadow]](https://gitlab.com/cs-shadow) on Jan 15, 2019, 00:19

## Summary

In our CI process, we explicitly run the tests as a normal user, to test for permission issues. This is perfectly fine but now we have tests that fail when run as the root user, as noted in https://gitlab.com/BuildStream/buildstream/merge_requests/1038#note_129667120.

Is this something that we say is expected, or should we attempt to patch the tests such that it is skipped when run as root?

Review the failing tests and permission-related discussion linked from GitLab merge request 1038, then reproduce the test run as root and as a normal user. Determine whether root execution is unsupported or whether the affected tests should be skipped, and document or verify the chosen behavior in the test suite.

Written by the indexing model from the issue text.