Better path handling in Python tests

This fixed issues where these would not run correctly on Windows.

Change-Id: I931f7009305262efa4b66d573de4ecffe93b9e5d
Reviewed-on: http://review.couchbase.org/32694
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>