CBQE-0: fix noisy "OSError: [Errno 10] No child processes"

to skip printout exception at the end of execution

Traceback (most recent call last):
  File "./testrunner", line 467, in <module>
    watcher()
  File "./testrunner", line 456, in watcher
    os.wait()
OSError: [Errno 10] No child processes

Change-Id: I39ed69cb058f4c6f4b6d5ad449eb8c80b452b049
Reviewed-on: http://review.couchbase.org/37798
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Tommie McAfee <tommie@couchbase.com>