Don't use !indisvalid indexes for conflict resolution.

[nmisch]

When reading !indisready indexes, their structural defects may elicit low-level errors, as seen in the new test coverage. While we must skip those, !indisvalid indexes don't have that problem and could be better than no index. Nonetheless, follow the server's infer_arbiter_indexes() in skipping !indisvalid indexes.

The test change complicates an existing test rather than setting up a new one. That saves a bit of test code and runtime. If you're prefer greater separation, let me know.