Adjust perl unveil test to test the right thing

Adjust perl unveil test to test the right thing

Normal users can't write to /dev/random, so the -w test would always
fail for root and succeed for everyone else no matter what unveil
did.  Testing with a temp file at least gives consistent results
no matter the user, even if I don't completely understand why the
-w and -r test results are different.

Noticed by bluhm@