Improve test a bit, now that we have fstat(2) support.

Improve test a bit, now that we have fstat(2) support.
The test was support to check if SUID/SGID bits are removed on first
write, but actually we were checking if they were removed after close.
Now we can check if SUID/SGID bits are gone after first write.

While here add checks to see if when both SUID and SGID bits are set they are
both cleared on first write.