tests: conditionalize tests based on presence of revlogs for files ~85 tests don't like our non-revlog file store for various reasons. This commit introduces hghave functionality for declaring and querying repository features. By default, we assume repositories have revlog-based file storage. But if the HGREPOFEATURES environment variable is set, we can override the default set of repository features. If you run the test harness with our simplestorerepo extension and an environment variable set to the proper value, you can override the hghave defaults to agree with simplestorerepo's version of reality. Various tests have been modified so behavior dependent on revlog-based file storage is marked as such. This fixes a handful of test failures with our custom file storage extension. But dozens remain. The point of this commit is to demonstrate how tests will need to be modified to account for custom storage implementations. TBH, I'm not convinced hghave is the proper layer for repository feature detection. I /think/ we'll eventually want something in run-tests.py itself. But that would require inventing a new primitive in the test harness. This is all very alpha at the moment. So I think hghave is an acceptable place to hang this feature detection. I think the right time to be thinking about integrating this into run-tests.py is *after* we have a stable alternate storage implementation in core. For now, let's try to make progress towards the idea of an alternate storage backend. Differential Revision: https://phab.mercurial-scm.org/D3030