Steps to reproduce :

Open attached project
Open TestRunner window
Expected : Tests using [TestFixture] or [TestFixtureSource] attribute should show up and work properly in TestRunner window.

Results :

Only the test "IsGreaterThenFive_OneFixture" is shown.
"IsGreaterThenFive_FixtureList" (uses 2 [TestFixture]) doesn't show
"IsGreaterThenFive_FixtureSource" (uses [TestFixtureSource] with 10 values) only shows one test that doesn't work properly.

Probably related - if you're using a parametrized test fixture and you try to specify multiple cases using multiple `[TestFixture]` attributes, then the tests also do not show up.

Conclusion
Nunit does not handle TestFixture or TestFixtureSource well when the class is not in a class.
If the test class in a class this works. The logic here is inside the Nunit framework.
By a result from this we are adding default namespace in the "Create -> Test -> Test Script" class.