CBQE-0: fix syntax error running testrunner.py

jenkins jobs failing:

File "./testrunner", line 2
  exec python ${0}.py "$@"
                      ^

because testrunner is a bash wrapper, this change points
to the actual python file

Change-Id: Iad416388724de611a996c59b512dfec8f03ee50d
Reviewed-on: http://review.couchbase.org/38323
Reviewed-by: Aruna Piravi <aruna@couchbase.com>
Tested-by: Aruna Piravi <aruna@couchbase.com>