MB-7467: added mapreduce test of context usability after timeout

Change-Id: I977b997e70c1b052ee939ca81317094e9b58769c
Reviewed-on: http://review.couchbase.org/23584
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-on: http://review.couchbase.org/24950