Change directory for py27 xdist-related envs

[nicoddemus]

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:

pytest/_pytest/python.py

Lines 48 to 49 in 111d640

	p = py.path.local(entry.path)
	return p != cutdir1 and not p.relto(cutdir2) and not p.relto(cutdir3)

py.path.local(entry.path) will create a full path using cwd because entry.path is relative, and cwd would point to the testdir directory (as it should).

I debugged up until Code.__init__:

pytest/_pytest/_code/code.py

Lines 19 to 28 in 111d640

	class Code(object):
	""" wrapper around Python code objects """
	def __init__(self, rawcode):
	if not hasattr(rawcode, "co_filename"):
	rawcode = getrawcode(rawcode)
	try:
	self.filename = rawcode.co_filename
	self.firstlineno = rawcode.co_firstlineno - 1
	self.name = rawcode.co_name

And I verified that the rawcode.co_filename is indeed a relative path when executed with xdist and a full path in non-distributed, but I don't have a clue as to why.

By running the tests in a directory other than where _pytest is, then rawcode.co_filename is always a full path and everything works.

I tried to back trace to other versions (pytest 3.1.0, execnet 1.4 and pytest-xdist 1.19) but the problem still happens in those versions, so I can't figure out why this suddenly started to fail.

I thought I would at least open a PR with my findings and a "workaround" to see if people have any suggestions.

Fix #2843

[coveralls]

Coverage remained the same at 92.244% when pulling c3ba922 on nicoddemus:fix-py27-xdist-envs-2843 into 111d640 on pytest-dev:master.

[coveralls]

Coverage remained the same at 92.244% when pulling 27cea34 on nicoddemus:fix-py27-xdist-envs-2843 into 111d640 on pytest-dev:master.

[RonnyPfannschmidt]

ok to merge, but needs a followup because it should work as is