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

@@ -1,41 +1,44
   $ hg init a
   $ cd a
   $ hg init b
   $ echo x > b/x
 
 Should print nothing:
 
   $ hg add b
   $ hg st
 
+  $ echo y > b/y
+  $ hg st
+
 Should fail:
 
   $ hg st b/x
   abort: path 'b/x' is inside nested repo 'b' (glob)
   [255]
   $ hg add b/x
   abort: path 'b/x' is inside nested repo 'b' (glob)
   [255]
 
 Should fail:
 
   $ hg add b b/x
   abort: path 'b/x' is inside nested repo 'b' (glob)
   [255]
   $ hg st
 
 Should arguably print nothing:
 
   $ hg st b
 
   $ echo a > a
   $ hg ci -Ama a
 
 Should fail:
 
   $ hg mv a b
   abort: path 'b/a' is inside nested repo 'b' (glob)
   [255]
   $ hg st
 
   $ cd ..