tests: add TestDiskFull

Also fix incomplete uid restoration in TestSupplementaryGroups
and replace syscall.Setregid and friends with unix.Setregid and
friends.

This test is added to check if have problems handling a full disk.
The ticket https://github.com/rfjakob/gocryptfs/issues/550 states
that the full disk was not where the backing gocryptfs filesystem
was, but this has no effect on gocryptfs, so we test the harder
case.

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