search: reduce boilerplate for indexed repohasfile tests

Created by: keegancsmith

Best reviewed commit by commit.

This PR greatly reduces the boilerplate for the indexed repohasfile tests. This is motivated by finding it hard to understand and extend the current tests. Additionally it will give us the power to reproduce a current issue in the implementation. Most of the commits are pretty straight-forward, but the first is the most interesting. Instead of using hardcoded responses, we create a simple zoekt searcher which can evaluate the queries we expect from repohasfile filtering.

Part of https://github.com/sourcegraph/sourcegraph/issues/7380