Make Node.warn support two forms, new and deprecated

As suggested during review, it now accepts two forms:

Node.warn(warning_instance)  (recommended)

Node.warn(code, message)  (deprecated)

src/_pytest/pytester.py

@@ -126,7 +126,7 @@ class LsofFdLeakChecker(object):
             error.append(error[0])
             error.append("*** function %s:%s: %s " % item.location)
             error.append("See issue #2366")
-            item.std_warn(pytest.PytestWarning("\n".join(error)))
+            item.warn(pytest.PytestWarning("\n".join(error)))
 
 
 # XXX copied from execnet's conftest.py - needs to be merged