Fix two failing tests after ATF update

[arichardson]

Summary

Since https://github.com/freebsd/atf/commit/4581cefc1e3811dd3c926b5dd4b15fd63d2e19da
ATF opens the results file on startup. This fixes problems like
capsicumized tests not being able to open the file on exit.

However, this test closes all file descriptors just to check that
socketpair returns fd 3+4 and thereby also closes the ATF results file.
This then results in an EBADF when writing the result so the test is
reported as broken.

While system calls that create new file descriptors (must?) use the lowest
available file descriptor number, it does not seem useful to test this
property here. Drop the check for FD==3/4 to unbreak the testsuite.

We could also try to re-open the results file in ATF if we get a EBADF
error, but that will fail when running under Capsicum.

Test Plan

tests pass now.

[arichardson]

Note: this also affects contrib/netbsd-tests/lib/libc/gen/posix_spawn/t_fileactions.c:t_spawn_fileactions, but I haven't tried fixing that one yet.

[cem]

The analysis makes sense and the fix looks fine to me. You could allocate two fds and check socketpair against both (instead of just low) in the final test, but I don't think there's much marginal value in that. (Frankly, I'm not a fan of unix' must-assign-lowest-fd behavior in general, but we have to abide by it.)