tests: fix test failure on vfat

It can't handle filenames with spaces. We don't have a good hghave
test for this, so we wrap it with unix-permissions.

tests/test-mq-qnew.t

@@ -302,6 +302,8 b' Test saving last-message.txt'
   > pretxncommit.unexpectedabort =
   > EOF
 
+#if unix-permissions
+
 Test handling default message with the patch filename with tail whitespaces
 
   $ cat > $TESTTMP/editor.sh << EOF
@@ -321,3 +323,5 b' Test handling default message with the p'
   # Parent 0000000000000000000000000000000000000000
 
   $ cd ..
+
+#endif