Change directory for py27 xdist-related envs

The "filter_traceback" function was not filtering the frames
that belonged to the pytest internals.

"filter_traceback" was receiving *relative* paths when running with
xdist, and full paths in non-distributed runs; for this reason
the traceback function did not consider the received path to be
relative to the pytest internal modules.

Fix #2843

tox.ini

@@ -64,8 +64,9 @@ deps =
     mock
     nose
     hypothesis>=3.5.2
+changedir=testing
 commands =
-    pytest -n1 -rfsxX {posargs:testing}
+    pytest -n1 -rfsxX {posargs:.}
 
 [testenv:py36-xdist]
 deps = {[testenv:py27-xdist]deps}
@@ -91,10 +92,11 @@ deps =
     pytest-xdist>=1.13
     hypothesis>=3.5.2
 distribute = true
+changedir=testing
 setenv =
     PYTHONDONTWRITEBYTECODE=1
 commands =
-    pytest -n3 -rfsxX {posargs:testing}
+    pytest -n3 -rfsxX {posargs:.}
 
 [testenv:py27-trial]
 deps = twisted