Make the runTest argument actually work.

testtools/testcase.py

@@ -86,6 +86,7 @@ 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)
         unittest.TestCase.__init__(self, *args, **kwargs)
         self._cleanups = []
         self._unique_id_gen = itertools.count(1)
@@ -95,7 +96,7 @@ class TestCase(unittest.TestCase):
         # __details is lazy-initialized so that a constructed-but-not-run
         # TestCase is safe to use with clone_test_with_new_id.
         self.__details = None
-        self.__RunTest = kwargs.get('runTest', RunTest)
+        self.__RunTest = runTest
         self.__exception_handlers = []
         self.exception_handlers = [
             (self.skipException, self._report_skip),

testtools/tests/test_runtest.py

@@ -8,6 +8,7 @@ from testtools import (
     TestCase,
     TestResult,
     )
+from testtools.matchers import Is
 from testtools.tests.helpers import ExtendedTestResult
 
 
@@ -176,6 +177,30 @@ class TestRunTest(TestCase):
             ], result._events)
 
 
+class CustomRunTest(RunTest):
+
+    def __init__(self, marker, *args, **kwargs):
+        super(CustomRunTest, self).__init__(*args, **kwargs)
+        self._marker = marker
+
+    def run(self, result=None):
+        return self._marker
+
+
+class TestTestCaseSupportForRunTest(TestCase):
+
+    def test_pass_custom_run_test(self):
+        class SomeCase(TestCase):
+            def test_foo(self):
+                pass
+        result = TestResult()
+        marker = object()
+        case = SomeCase(
+            'test_foo', runTest=lambda *args: CustomRunTest(marker, *args))
+        from_run_test = case.run(result)
+        self.assertThat(from_run_test, Is(marker))
+
+
 def test_suite():
     from unittest import TestLoader
     return TestLoader().loadTestsFromName(__name__)