Fixed dummy runTest on functional test case. (#90)

In functional doc tests you can apparently have a test case that has
no runTest method.  Until now the Testing package added a dummy
runTest method in that case, and set it to None.

But when this dummy runTest method gets called, you get an error:

  Error in test runTest (Testing.ZopeTestCase.ZopeTestCase.FunctionalTestCase)
  Traceback (most recent call last):
    File ".../lib/python2.7/unittest/case.py", line 329, in run
      testMethod()
  TypeError: 'NoneType' object is not callable

Simply importing Testing.ZopeTestCase.FunctionalTestCase in a new test
file may be enough to trigger this.

So this has something to do with the order in which tests are found.

I fixed it by making the dummy runTest method callable.

src/Testing/ZopeTestCase/zopedoctest/functional.py

@@ -254,8 +254,9 @@ class ZopeSuiteFactory(object):
 
         # If the test_class does not have a runTest method, we add
         # a dummy attribute so that TestCase construction works.
+        # runTest may get called, so set it to something that works.
         if not hasattr(test_class, 'runTest'):
-            setattr(test_class, 'runTest', None)
+            setattr(test_class, 'runTest', lambda x: None)
 
         # Create a TestCase instance which will be used to execute
         # the setUp and tearDown methods, as well as be passed into