Support internal test cases which are not registered

* This is useful for large tests like Mesh_Zoo and Texture_Zoo which are API
  independent, which can derive from TestCase to get all the helpers, but then
  not get auto-added until an API-specific test derives from them

util/test/rdtest/runner.py

@@ -21,7 +21,7 @@ def get_tests():
     for m in sys.modules.values():
         for name in m.__dict__:
             obj = m.__dict__[name]
-            if isinstance(obj, type) and issubclass(obj, testcase.TestCase) and obj != testcase.TestCase:
+            if isinstance(obj, type) and issubclass(obj, testcase.TestCase) and obj != testcase.TestCase and not obj.internal:
                 testcases.append(obj)
 
     testcases.sort(key=lambda t: (t.slow_test,t.__name__))

util/test/rdtest/testcase.py

@@ -128,6 +128,7 @@ class ConstantBufferChecker:
 
 class TestCase:
     slow_test = False
+    internal = False
     demos_test_name = ''
     demos_frame_cap = 5
     _test_list = {}