ref #322 cleanup all teardown calling to only happen when setup succeeded.

don't use autouse fixtures for now because it would cause a proliferation
and overhead for the execution of every test.  Rather introduce a
node.addfinalizer(fin) to attach a finalizer to the respective node
and call it from node.setup() functions if the setup phase succeeded
(i.e. there is no setup function or it finished successfully)

testing/python/fixture.py

@@ -834,8 +834,6 @@ class TestFixtureUsages:
         l = reprec.getfailedcollections()
         assert len(l) == 1
 
-    @pytest.mark.xfail(reason="unclear if it should be supported at all, "
-                              "currently broken")
     def test_request_can_be_overridden(self, testdir):
         testdir.makepyfile("""
             import pytest