Don't test that errno is still 0 after POSIX::f?pathconf

Don't test that errno is still 0 after POSIX::f?pathconf

I think the best we can do with respect to the f?pathconf tests is to
make sure that the perl call doesn't die, and that the system call
doesn't fail.  And it's arguable we should only be testing the former.
But since we've been testing more that this anyway, it's probably safe
to test both.

With respect to the sysconf call, I think we shouldn't test more than
that perl doesn't die.  Any further testing would require different
tests based the argument being passed in.  Before doing that, it's
probably worth considering the purpose of the tests.  I don't think we
really want to test that POSIX has been implemented correctly, only that
our layer over it is correctly implemented.

This fixes RT #112866.