Enable test parallelization

[tomasaschan]

Since xUnit 2.3 runs more stuff in parallel, I had to explicitly disable parallel runs of the EF tests since they re-use the same db instance.

It might be worthwhile to re-vamp the test fixture and base class for EF tests to create a new db instance for each test to enable parallel runs again, and then see if parallel runs or re-using the db instance provides the larger performance gain.