Added warns to assert warnings are thrown

Works in a similar manner to `raises`, but for warnings instead
of exceptions. Also refactored `recwarn.py` so that all the
warning recording and checking use the same core code.

testing/python/raises.py

@@ -34,7 +34,6 @@ class TestRaises:
             raise BuiltinAssertionError
         """)
 
-    @pytest.mark.skipif('sys.version < "2.5"')
     def test_raises_as_contextmanager(self, testdir):
         testdir.makepyfile("""
             from __future__ import with_statement