CI: rewrite signal tests

[apostasie]

Fix #3908

Currently, tests involving signals have issues:

• bash sig proxy code is duplicated
• tests are flaky, as they may not wait for the container to start or the trap to be set - and/or use arbitrary sleep waits
• a couple of these tests only test half of what they should (if the shell script exits when trapping the signal, we fail to test that the container should exit or restart on its own, and not through the script exiting its loop)

This PR does:

• expose the ability to send a signal to the tested command (in test tooling)
• rewrite the SigProxy helper with the new test framework and expanded abilities
• rewrite the tests themselves, fixing the aforementioned shortcomings

Hopefully these tests should stop flaking out (assuming log will be reliable in these cases).

Note that this PR likely conflicts with other in-flight PR impacting test tooling (specifically #3889).
Depending on merge order, I will rebase of course.

[apostasie]

Rebased.

[AkihiroSuda]

Thanks