GitHub CI: run most tests as a non-privileged user.

GitHub CI: run most tests as a non-privileged user.

We don't really care about the throwaway container running in a throwaway
VM, but it's still better to run tests that do not require elevated
privileges as a normal user, at least to be sure that the ability to do
this is working.

Also, some tests (like the new command-fsck.test) can perform more checks
with a restricted user account.