Get mkdir.t working under miniperl *again*

In commit ca7cc9e26e, I got mkdir.t to pass under miniperl when
Errno.pm has not been generated yet.

The way I did it, though, causes the test to fail under miniperl if
Errno.pm *has* been generated.  The ‘require’ call that tests to
see whether Errno can load ends up resetting $! before we test the
value in %!.

Also, we don’t need require inside a string eval.