Adjust tests in case running as root

[mwichmann]

Although validation tests are not normally run as root, there may be circumstances when it happens - one known case is when the test suite is run as part of a particular Linux distros package construction. It isn't too hard to avoid the few places where we counted on something failing because of permissions, which don't if the user is root - added a few skips.

This is a test-only change, no impact on SCons or docs.

Contributor Checklist:

• [X] I have created a new test or updated the unit tests to cover the new/changed functionality.
• [X] I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• [ ] I have updated the appropriate documentation