Since Unity 5.6 and the new runner, NUnit tests contained within custom assemblies are no longer able to be used by the runner.

To repro:
1) Open attached project
2) See in Test Runner Window that only the tests in the two CS scripts are shown. It should instead also show the tests contained within the DLLs.

More info:

- MorePlayModeTests.dll contains more play mode tests: MorePlayModeTests.Tests.PlayModeTestsSimplePasses and MorePlayModeTests.Tests.PlayModeTestsWithEnumeratorPasses

- MoreEditModeTests.dll contains MoreEditModeTests.Tests.AnotherEditModeTestsSimplePasses and MoreEditModeTests.Tests.AnotherEditModeTestsWithEnumeratorPasses

- NoPlayModeTests.dll contains no tests. Contains Clicker MonoBehaviour class and a Frog class (not MonoBehaviour).

- NoEditModeTests.dll contains no tests. Contains DatabaseChecker class with CheckDatabase method, MenuItem ("Example/Check Database").

There are also two scripts containing tests. PlayModeTests.cs and EditModeTests.cs.

Only the tests from the two cs scripts show in the Test Runner.

.mdb files are included for each DLL.