Restore exception context when passed in loader's _makeFailedTest method

Loader's `_makeFailedTest` now handles both the case where `exception` is an
Exception instance and the case where `exception` is an exception information
tuple (as given by `sys.exc_info()`). Rely on six.reraise to handle the latter
case.

Closes nose-devs#48.

nose2/loader.py

@@ -7,6 +7,8 @@
 import logging
 import traceback
 
+import six
+
 from nose2 import events
 from nose2.compat import unittest
 
@@ -114,7 +116,11 @@ def discover(self, start_dir=None, pattern=None):
 
     def _makeFailedTest(self, classname, methodname, exception):
         def testFailure(self):
-            raise exception
+            if isinstance(exception, Exception):
+                raise exception
+            else:
+                # exception tuple (type, value, traceback)
+                six.reraise(*exception)
         attrs = {methodname: testFailure}
         TestClass = type(classname, (unittest.TestCase,), attrs)
         return self.suiteClass((TestClass(methodname),))