test: make sigint test to actually check child pid

[axxie]

The aim of this PR is to fix some issues in SIGINT test.

The problem

Due to combination of the following effects, the SIGINT test does not actually test what it is made for:

1. The callback of string.replace method takes the child pid from incorrect parameter:
   

   nodemon/test/misc/sigint.test.js

   Lines 16 to 22 in cd45d74

   	output: function (data) {
   	if (match(data, 'pid: ')) {
   	data.replace(/pid: (\d+)/, function (m) {
   	childPID = m;
   	});
   	}
   	},

   
   According to MDN, the first parameter contains entire substring, not the first capture group. As the result, the childPID contains string, not just number (“pid: 1234”).
2. The test application ignores SIGINT signal and keeps hanging indefinitely:
   

   nodemon/test/fixtures/sigint.js

   Lines 1 to 3 in cd45d74

   	process.on('SIGINT', function() {
   	// do nothing here
   	});

   
   In fact, you can see it in the process list after running tests even if all tests pass.
3. However, the check for child not being running after nodemon exit incorrectly succeeds:
   

   nodemon/test/misc/sigint.test.js

   Lines 37 to 42 in cd45d74

   	try {
   	process.kill(childPID, 0);
   	assert(false, 'child is still running at ' + childPID);
   	} catch (e) {
   	assert(true, 'child process was not running');
   	}

   
   This happens because childPID is string and process.kill requires number and throws an exception. Due to exception, the test code assumes the child is not running and test passes.

Tested on:

• nodemon: 2.0.2 (from sources in master)
• node -v: v12.14.0
• Operating system/terminal environment: Ubuntu 18.04.3 LTS

The fix

I implemented the following fixes:

1. Changed the test application to only ignore the SIGINT if --dont-exit parameter is passed. In all other cases it exits upon receiving SIGINT as any normal application would do.
2. Fixed the parsing of the child pid from the string
3. Implemented more precise checking of the running process
4. Split the test into two: one with child exiting on SIGINT (fast) and another one with child hanging and nodemon still exiting after 10 seconds.

[stale]

This issue has been automatically marked as idle and stale because it hasn't had any recent activity. It will be automtically closed if no further activity occurs. If you think this is wrong, or the problem still persists, just pop a reply in the comments and @remy will (try!) to follow up.
Thank you for contributing <3

[axxie]

Actually, this is just a preparation for another fix, for more serious issue.
This one is still an issue.