Cosmos: improve testing speed

It's likely that we have certain tests which take a very long time (possibly anything needing to create databases/containers), but the majority of tests seem to run very fast (e.g. all query tests combined are a small fraction of the total assembly running time). We should find out which tests are the worst offenders and see if there's something we can do about them.

Also, look into enabling test parallelization (this is currently disabled via `[assembly: CollectionBehavior(DisableTestParallelization = true)]`). As above, it's likely we have some tests which cannot be parallelized, but we do it for those tests only rather than disabling for the entire assembly.