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>
diff --git a/t/t0066-dir-iterator.sh b/t/t0066-dir-iterator.sh
index 9354d3f..92910e4 100755
--- a/t/t0066-dir-iterator.sh
+++ b/t/t0066-dir-iterator.sh
@@ -55,13 +55,13 @@
 test_expect_success 'begin should fail upon inexistent paths' '
 	test_must_fail test-tool dir-iterator ./inexistent-path \
 		>actual-inexistent-path-output &&
-	echo "dir_iterator_begin failure: 2" >expected-inexistent-path-output &&
+	echo "dir_iterator_begin failure: ENOENT" >expected-inexistent-path-output &&
 	test_cmp expected-inexistent-path-output actual-inexistent-path-output
 '
 
 test_expect_success 'begin should fail upon non directory paths' '
 	test_must_fail test-tool dir-iterator ./dir/b >actual-non-dir-output &&
-	echo "dir_iterator_begin failure: 20" >expected-non-dir-output &&
+	echo "dir_iterator_begin failure: ENOTDIR" >expected-non-dir-output &&
 	test_cmp expected-non-dir-output actual-non-dir-output
 '