* Fix timing issue in singleton fixture.
Singleton now sends the "we've started" message out only after it's
received a `'ready'` event from `app`. Previously it sent the message
out immediately, resulting in the parent test trying to manipulate it
before Singleton's event loop was fully bootstrapped.
* Check for graceful exits on Linux, too.
Rewrite the "exits gracefully on macos" spec to run on Linux too.
* Check for graceful exits everywhere.
* Tweak comment
* Better error logging in api-app-spec.js. (#12122)
In the 'exits gracefully' test for app.exit(exitCode),
print the relevant error information if the test fails.
* Run the exit-gracefully test on macOS and Linux.
Windows does not support sending signals, but Node.js offers some
emulation with process.kill(), and subprocess.kill(). Sending signal 0
can be used to test for the existence of a process. Sending SIGINT,
SIGTERM, and SIGKILL cause the unconditional termination of the target
process.
So, we'll need a different approach if we want to test this in win32.
