tests: test-pathconflicts-merge.t requires symlinks

Once we're ready to turn this functionality on more widely, we might
want to write a symlink-free version of the test that can run on
Windows, but for now we'll just leave it disabled there.

Differential Revision: https://phab.mercurial-scm.org/D1710

tests/test-pathconflicts-merge.t

@@ -1,3 +1,5 b''
+#require symlink
+
 Path conflict checking is currently disabled by default because of issue5716.
 Turn it on for this test.