Add "kill" command to terminate background exec

[kortschak]

I am using testscript to test the behaviour of a program that is not intended to terminate in production. I would like to examine its output while running for a period, but it is not possible to obtain stdout/stderr until a background exec has terminated. To achieve this I would like to be able to

cmd &name&
sleep 1s
kill name
wait name # depending on implementation of kill this may or may not be necessary.

stdout <pattern>
stderr <pattern>

I am able to add the sleep, but access to the exec's termination details are not exposed, so the script cannot terminate the exec unless invoking stop, which would defeat the test.

[ldemailly]

is that covering #195 too?

[kortschak]

I didn't see that. Yes, it would cover that.