highlighting: add support for simulating highlighting timeouts
Created by: slimsag
This is useful for confirming our behavior of highlighting timeouts, and can be used in the future to automate testing of this behavior as part of an e2e test.
Primarily adding to reproduce https://github.com/sourcegraph/sourcegraph/issues/4267
Note: This matches other cases where e.g. github.com/sourcegraphtest/AlwaysCloningTest is a special repository for testing gitserver cloning states end-to-end.
To use this you just need to add github.com/sourcegraph/AlwaysHighlightTimeoutTest to your instance and view a file in there or search results coming from that repository.
Helps #4267