Add tests for FractalSlurmSSHExecutor.shutdown

[tcompa]

What we are specially interested in is whether shutdown may fail because it is triggered at the same time when some other SSH command is in progress.

[tcompa]

We debugged (with @mfranzon) the corresponding bug, and the motivation is that shutdown was triggered before any job had been submitted. This bug affects both sudo/ssh SLURM executor, but it's much more relevant for the SSH one because a much longer time passes between the job-submission API call and the actual job submission (in the executor and on SLURM). During this time, a shutdown would corrupt the executor, by closing its auxiliary thread.

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Here is the timeline of the observed bug

19:54:13.5 -> job-submit API call
19:54:16.5 -> end of SSH handshake
19:54:16.5 -> many small things
19:54:16.5 -> start remote extract archive
19:54:19.5 -> shutdown, closes waiting thread
19:54:24.9 -> start remote extract archive
19:54:24.9 -> sbatch is successful
19:56:12.x -> python task fails (no actions triggered)
19:56:12.x -> SLURM job is complete (no actions triggered)

The reason why SLURM job completion does not trigger any action is that the waiting thread is already closed.

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Here is how the timeline should have looked

19:54:13.5 -> job-submit API call
19:54:16.5 -> end of SSH handshake
19:54:16.5 -> many small things
19:54:16.5 -> start remote extract archive
19:54:19.5 -> shutdown, closes waiting thread AND CLOSES EXECUTOR