introduce an option that avoids discovery of classes other than unittest.TestCase in modules

importing unittest.

pytest/plugin/python.py

@@ -59,6 +59,8 @@ def pytest_pycollect_makeitem(__multicall__, collector, name, obj):
     if res is not None:
         return res
     if collector._istestclasscandidate(name, obj):
+        if hasattr(collector.obj, 'unittest'):
+            return # we assume it's a mixin class for a TestCase derived one
         return Class(name, parent=collector)
     elif collector.funcnamefilter(name) and hasattr(obj, '__call__'):
         if is_generator(obj):
@@ -124,6 +126,7 @@ class PyobjMixin(object):
         return self._fslineno
 
     def reportinfo(self):
+        # XXX caching?
         obj = self.obj
         if hasattr(obj, 'compat_co_firstlineno'):
             # nose compatibility