Merge _pytest fixture reintroduction from pytest-2.7 branch

This was accidentally removed while some plugins depend on it.

CHANGELOG

@@ -44,6 +44,8 @@
 - fixed regression to 2.6.4 which surfaced e.g. in lost stdout capture printing
   when tests raised SystemExit. Thanks Holger Krekel.
 
+- reintroduced _pytest fixture of the pytester plugin which is used
+  at least by pytest-xdist.
 
 2.7.0 (compared to 2.6.4)
 -----------------------------

_pytest/pytester.py

@@ -15,6 +15,24 @@ from _pytest.core import HookCaller, add_method_wrapper
 
 from _pytest.main import Session, EXIT_OK
 
+# used at least by pytest-xdist plugin
+@pytest.fixture
+def _pytest(request):
+    """ Return a helper which offers a gethookrecorder(hook)
+    method which returns a HookRecorder instance which helps
+    to make assertions about called hooks.
+    """
+    return PytestArg(request)
+
+class PytestArg:
+    def __init__(self, request):
+        self.request = request
+
+    def gethookrecorder(self, hook):
+        hookrecorder = HookRecorder(hook._pm)
+        self.request.addfinalizer(hookrecorder.finish_recording)
+        return hookrecorder
+
 
 def get_public_names(l):
     """Only return names from iterator l without a leading underscore."""