test-nested-repo.t: touch another file inside nested repo

This helped uncover a bug in a patchset I've been writing.

tests/test-nested-repo.t

@@ -8,6 +8,9 Should print nothing:
   $ hg add b
   $ hg st
 
+  $ echo y > b/y
+  $ hg st
+
 Should fail:
 
   $ hg st b/x