atf: Don't be deterred by weird umasks.

If the current umask is weird, ATF goes to great lengths to tell us that
it can't possibly work in these conditions, instead of just dealing with
it. This makes it unreasonably hard to use ATF to test how our own code
handles unusual umasks.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: igoro, kevans, ngie
Differential Revision: https://reviews.freebsd.org/D50267