test: fix flaky test-trace-sigint-on-idle

Previously, the test could fail on slow machines because the
child process was still in the process of starting up after
one second, and not yet idle.

To resolve this:
- Wait for a message from the child process indicating that it
  had started.
- Wait some time after that, but make it platform-dependent to
  account for timing differences.
- Remove the timer in the child process.

PR-URL: https://github.com/nodejs/node/pull/31645


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>