tests: add end-to-end tests checking process termination behavior

[vlaci]

• It runs only when e2e marker is passed (and in CI)
• current error cases are marked with xfail
• configured pytest to use strict xfail policy, failing xfail marked tests if they were to pass

In preparation for #1426, added a few end-to-end tests to assert on what behavior we want.

In a couple of sentences:

• SIGTERM and CTRL-C in terminal should promptly terminate all processes exiting with non-0 exit code
• Crashing handlers should terminate all processes exiting with non-0 exit code

For SIGINT handling, pexpect is used to simulate terminal behavior, when CTRL-C being propagated to all members of the foreground process group

[vlaci]

Re xpass tests in CI: It looks like there is some environment/python version specific behavior

[qkaiser]

Interesting behavior:

test	version	status
test_sigterm_terminates_promptly[single-process]	3.10	XFAIL
test_sigterm_terminates_promptly[single-process]	3.13	XFAIL
test_sigterm_terminates_promptly[multi-process]	3.10	XFAIL
test_sigterm_terminates_promptly[multi-process]	3.10	XFAIL
test_sigint_terminates_promptly[single-process]	3.10	PASSED
test_sigint_terminates_promptly[single-process]	3.13	FAILED
test_sigint_terminates_promptly[multi-process]	3.10	PASSED
test_sigint_terminates_promptly[multi-process]	3.13	PASSED
test_worker_crash_terminates_main_process[single-process]	3.10	PASSED
test_worker_crash_terminates_main_process[single-process]	3.13	PASSED
test_worker_crash_terminates_main_process[multi-process]	3.10	XFAIL
test_worker_crash_terminates_main_process[multi-process]	3.13	XFAIL

[qkaiser]

Cool !!