Add a class variable, run_tests_with, to allow controlling which runner

to run tests with.

testtools/testcase.py

@@ -74,10 +74,14 @@ class TestCase(unittest.TestCase):
     :ivar exception_handlers: Exceptions to catch from setUp, runTest and
         tearDown. This list is able to be modified at any time and consists of
         (exception_class, handler(case, result, exception_value)) pairs.
+    :cvar run_tests_with: A `RunTest` class to run tests with.  Defaults to
+        `RunTest`.
     """
 
     skipException = TestSkipped
 
+    run_tests_with = RunTest
+
     def __init__(self, *args, **kwargs):
         """Construct a TestCase.
 
@@ -86,7 +90,7 @@ class TestCase(unittest.TestCase):
             supplied testtools.runtest.RunTest is used. The instance to be
             used is created when run() is invoked, so will be fresh each time.
         """
-        runTest = kwargs.pop('runTest', RunTest)
+        runTest = kwargs.pop('runTest', self.run_tests_with)
         unittest.TestCase.__init__(self, *args, **kwargs)
         self._cleanups = []
         self._unique_id_gen = itertools.count(1)

testtools/tests/test_runtest.py

@@ -179,12 +179,10 @@ class TestRunTest(TestCase):
 
 class CustomRunTest(RunTest):
 
-    def __init__(self, marker, *args, **kwargs):
-        super(CustomRunTest, self).__init__(*args, **kwargs)
-        self._marker = marker
+    marker = object()
 
     def run(self, result=None):
-        return self._marker
+        return self.marker
 
 
 class TestTestCaseSupportForRunTest(TestCase):
@@ -194,11 +192,19 @@ class TestTestCaseSupportForRunTest(TestCase):
             def test_foo(self):
                 pass
         result = TestResult()
-        marker = object()
-        case = SomeCase(
-            'test_foo', runTest=lambda *args: CustomRunTest(marker, *args))
+        case = SomeCase('test_foo', runTest=CustomRunTest)
         from_run_test = case.run(result)
-        self.assertThat(from_run_test, Is(marker))
+        self.assertThat(from_run_test, Is(CustomRunTest.marker))
+
+    def test_default_is_runTest_class_variable(self):
+        class SomeCase(TestCase):
+            run_tests_with = CustomRunTest
+            def test_foo(self):
+                pass
+        result = TestResult()
+        case = SomeCase('test_foo')
+        from_run_test = case.run(result)
+        self.assertThat(from_run_test, Is(CustomRunTest.marker))
 
 
 def test_suite():