test-dir-iterator: do not assume errno values

test-dir-iterator: do not assume errno values

A few tests printed 'errno' as an integer and compared with
hardcoded integers; this is obviously not portable.

A two things to note are:

 - the string obtained by strerror() is not portable, and cannot be
   used for the purpose of these tests.

 - there unfortunately isn't a portable way to map error numbers to
   error names.

As we only care about a few selected errors, just map the error
number to the name before emitting for comparison.

Reported-by: Randall S. Becker <rsbecker@nexbridge.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>