Some tests fail when run with superuser privileges

[exarkun]

• test_sign_bad_perms
• test_unreadable_config
• test_private_config_unreadable
• test_private_config_unreadable_preexisting
• test_write_config_unwritable_file

These all assume that the test process won't be able to read or write a path where the associated filesystem permission bit is not set. This is only true as long as the process isn't a superuser. If it is, it can do what it wants and the tests fail as a result.

It's not a great idea to run Tahoe-LAFS as a superuser but nothing we do actively _prevents_ you from doing so. Also, right now, it would be convenient to run some of the test jobs on CI as root inside a Docker container (otherwise we need to do an unknown amount of Docker image hacking to make running as another user possible).

So, skip these tests if the test process is root.

[GitHub <noreply@…>]

In a297d6be/trunk:

Merge pull request #1316 from exarkun/4049.skip-some-when-superuser

Skip some tests when run as superuser

Fixes ticket:4049