tests: fix portability issue with test-merge-tools stable

tests: fix portability issue with test-merge-tools

Not sure how this ever worked.

--- a/tests/test-merge-tools.t	Fri Apr 17 12:39:55 2015 -0400
+++ b/tests/test-merge-tools.t	Fri Apr 17 15:42:20 2015 -0500
@@ -105,9 +105,11 @@
   M f
   ? f.orig
 
+#if unix-permissions
+
 unexecutable file in $PATH shouldn't be found:
 
-  $ touch false
+  $ echo "echo fail" > false
   $ hg up -qC 1
   $ PATH="`pwd`:$BINDIR" $PYTHON "$BINDIR"/hg merge -r 2
   merging f
@@ -118,6 +120,8 @@
   [1]
   $ rm false
 
+#endif
+
 executable directory in $PATH shouldn't be found:
 
   $ mkdir false