Vusual Studio 2013 Test Explorer error

I have a .Net 4.5.2 solution developed in Visual Studio 2013 Pro, which I have been working on in VS2015 Enterprise. When I re-open the solution in VS2013 and try to run the tests, I get the following error in the test output pane:

------ Discover test started ------ The request has taken more than 10 seconds to respond, aborting it. Model compatibility cannot be checked because the database does not contain model metadata. Ensure that IncludeMetadataConvention has been added to the DbModelBuilder conventions. ========== Discover test finished: 482 found (0:00:39.906) ==========

and I can't run the tests. Also, Test Explorer fails to find and list the unit tests.

I've tried deleting the test results database files and letting VS2013 recreate them, but I still get the same error. I am not using EF in the solution, but Googling for the error message only yields a load of EF stuff.

What can I do to fix this? How do I "Ensure that IncludeMetadataConvention has been added to the DbModelBuilder convention"? I've never need to do anything explicit like that before.

TIA