Rename testrunner to testrunner.py

To fix problems loading the module on windows

Change-Id: I70c7b8931bc528eb8dc0becfcb34d6416e22aa7b
Reviewed-on: http://review.couchbase.org/37088
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>