tmux-commandline can fail with
```
prompt 4> commandline -i "echo $(printf %0"$COLUMNS"d)"
```
And I just can't even.
job_summary is annoyingly tight.
Also count cancel_event as a *skip*, not success.
Failed on Cirrus Alpine.
The only explanation I can come up with here is that this took over
100ms to start `true | sleep 6`.
The alternative is that it started it and then did not regain control
in 6 seconds to kill that sleep.
Part of #11036
On macOS, the tests would often fail because calls to `pkill` would "leak"
across tests: kill processes run by other tests. This is because on macOS,
the -P argument to pkill must come before the process name. On Linux it
doesn't matter.
This improves test reliability on Mac.
Might help figuring out where this times out on CI?
We're waiting *20 seconds* for the output to appear, there's no way
that's too slow. So maybe we're going too fast elsewhere?
It's useless - `expect` has a timeout anyway, and it defaults to 5s,
so these 0.5s sleeps just mean it'll always take at least 0.5s.
Sometimes it is useful to let things settle before *sending* text, and
it would be nice to be able to set the timeout for each expect
separately, but just adding to the timeout isn't useful.
