test-dirstate-version-fallback: future-proof the test for a different default

Dirstate-v2 will become the default at some point, which would cause this
test to fail. Let's save someone else the headache later.

tests/test-dirstate-version-fallback.t

@@ -1,6 +1,8 b''
   $ cat >> $HGRCPATH << EOF
   > [storage]
   > dirstate-v2.slow-path=allow
+  > [format]
+  > use-dirstate-v2=no
   > EOF
 
 Set up a v1 repo