Change directory for py27 xdist-related envs
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

pytest-dev/pytest,txomon/pytest,hackebrot/pytest,rmfitzpatrick/pytest,The-Compiler/pytest,alfredodeza/pytest,tomviner/pytest,nicoddemus/pytest,davidszotten/pytest,tareqalayan/pytest,markshao/pytest,RonnyPfannschmidt/pytest,The-Compiler/pytest,tomviner/pytest,pfctdayelise/pytest,ddboline/pytest,nicoddemus/pytest,Akasurde/pytest