CBQE-0: use os.sep in testrunner paths

instead of assuming '/' unix dir structure

Change-Id: I22cb18c4c8dd55b00a2024d05c5c62b32eed7e65
Reviewed-on: http://review.couchbase.org/36904
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: buildbot <build@couchbase.com>