tests: stabilize `test-largefiles-cache.t` on Windows The `undo.backup.dirstate.bck` was created by `hg commit -m 'add largefile'` at line 18, and deleted in the conditional block by `hg push -q http://localhost:$HGPORT/` at line 138. That's... surprising, but probably not harmful, and can be debugged from Linux if it is a problem. This was showing up in `find src/.hg/largefiles/* | grep -E "(dirstate|$hash)"`.