test: wait for a delivered SIGINT instead of re-pressing on a timer (#63597)
The "SIGINT to a non-interactive process blocked in sleep" test pressed
^C up to three times, 10 seconds apart, and required the child to have
exited within those 30 seconds. On loaded CI machines the interrupt does
arrive, but reporting it and exiting can take longer than that (the
failing runs show the child's InterruptException report on stderr after
the deadline), and every additional press can cancel the report that is
already in progress.
Resend SIGINT only until the interrupt visibly arrives - the child starts
writing to stderr, or exits - and then give it a generous deadline to
finish exiting without further presses. Also run the child with the bare
executable like the REPL interrupt tests, rather than with the suite's
`julia_cmd()` flags (`--check-bounds=yes` etc.), which only add
compilation work to the report path and are not what this test probes,
and stop waiting on the stderr reader for a child that never exited.
https://buildkite.com/julialang/julia-pr/builds/3268#01a0fd10-632f-492a-842d-c51b11b88115
Assisted-by: Claude Code (Opus 5.5)