fix issue91 introduce new py.test.xfail(reason) helper

to imperatively mark a test as expected to fail. Can
be used from within setup and test functions. This is
useful especially for parametrized tests when certain
configurations are expected-to-fail.  In this case the
declarative approach with the @py.test.mark.xfail cannot
be used as it would mark all configurations as xfail.

--HG--
branch : trunk

py/_plugin/pytest_skipping.py

@@ -185,9 +185,17 @@ def pytest_runtest_setup(item):
 def pytest_runtest_makereport(__multicall__, item, call):
     if not isinstance(item, py.test.collect.Function):
         return
-    evalxfail = getattr(item, '_evalxfail', None)
-    if not evalxfail:
-        return
+    if not (call.excinfo and 
+        call.excinfo.errisinstance(py.test.xfail.Exception)):
+        evalxfail = getattr(item, '_evalxfail', None)
+        if not evalxfail:
+            return
+    if call.excinfo and call.excinfo.errisinstance(py.test.xfail.Exception):
+        rep = __multicall__.execute()
+        rep.keywords['xfail'] = "reason: " + call.excinfo.value.msg
+        rep.skipped = True
+        rep.failed = False
+        return rep
     if call.when == "setup":
         rep = __multicall__.execute()
         if rep.skipped and evalxfail.istrue():