Filter to only show tests that have associated validation results

The GET endpoints for validation tests carry no information about associated results, so a client wanting to show only tests that have been run must call the results endpoint once per test.

Add a counter to the test GET endpoints giving the number of associated results across all instances of that test, and a validated filter on /tests/, mirroring what /models/ already provides.

Originally raised for both models and tests as HumanBrainProject/hbp-validation-framework#333. The models half is done — /models/ returns validation_count and accepts validated=true — so only the tests half remains. Note that the model counter is filled from the stored KG query, so the VF_ValidationTest query will probably need updating as well as the API code.