Test execution is currently sequential.

To enable parallel tests, we likely want to address the outermost loop:

foreach (var testClass in convention.Classes.Filter(candidateTypes))

Originally I wrote here that the offenders are the foreach within CreateInstancePerCase and ExecuteCases, but I think we don't want to parallelize within a given test class. There are too many complications parallelizing intra-test-class.

Should parallelization be enabled for the whole run ("from the outside") or explicitly within convention classes?