document and refine/rename item **runtest** hook invocations

capture output separately for fixture and actual test run

--HG--
branch : trunk

py/test/plugin/pytest_xfail.py

@@ -10,11 +10,13 @@ example:
 """
 import py
 
-def pytest_runtest_makereport(__call__, item, excinfo, when, outerr):
+def pytest_runtest_makereport(__call__, item, call):
+    if call.when != "call":
+        return
     if hasattr(item, 'obj') and hasattr(item.obj, 'func_dict'):
         if 'xfail' in item.obj.func_dict:
             res = __call__.execute(firstresult=True)
-            if excinfo:
+            if call.excinfo:
                 res.skipped = True
                 res.failed = res.passed = False
             else: