Make SubRequest.addfinalizer an explicit method

This implicit definition really tripped me while debugging #2127,
unfortunately hidden as it was in the middle of all the variable
declarations.

I think the explicit definition is much easier for the eyes and IDEs
to find.

_pytest/fixtures.py

@@ -575,7 +575,6 @@ class SubRequest(FixtureRequest):
         self.param_index = param_index
         self.scope = scope
         self._fixturedef = fixturedef
-        self.addfinalizer = fixturedef.addfinalizer
         self._pyfuncitem = request._pyfuncitem
         self._fixture_values = request._fixture_values
         self._fixture_defs = request._fixture_defs
@@ -586,6 +585,9 @@ class SubRequest(FixtureRequest):
     def __repr__(self):
         return "<SubRequest %r for %r>" % (self.fixturename, self._pyfuncitem)
 
+    def addfinalizer(self, finalizer):
+        self._fixturedef.addfinalizer(finalizer)
+
 
 class ScopeMismatchError(Exception):
     """ A fixture function tries to use a different fixture function which