test-dirstate-race: ensure that a isn't in the lookup set at the end

We're going to rely on this in upcoming patches.

tests/test-dirstate-race.t

@@ -156,4 +156,4 b' treated differently in _checklookup() ac'
   $ hg files
   a
   $ hg debugdirstate
-  n * * * a (glob)
+  n 644          2 unset               a