test-dirstate: use new `--docket` flag to get the data uuid

This is more robust.

Differential Revision: https://phab.mercurial-scm.org/D12592

tests/test-dirstate.t

@@ -129,7 +129,7 b' valid.'
   > }
 
   $ find_dirstate_uuid () {
-  >   dirstate_data_files | sed 's#.hg/dirstate.##'
+  >   hg debugstate --docket | grep uuid | sed 's/.*uuid: \(.*\)/\1/'
   > }
 
   $ dirstate_uuid_has_not_changed () {