feat(server): add 'listening' event with port number

[karol-f]

Sometimes, when you want to use karma port programatically (e.g. to send manually browser to karma localhost: address), you face problem with not knowing exact port number.

Port number might be different from one set in config as sometimes you run more than one instances of karma. Below image show such situation:

This PR adds 'listening' event to karma. After this event you are sure that you have correct karma port number.

You may use it like this:

let server = new Server(karmaConfig, done);
server.once('listening', function (e, port) {
    console.log(port); //port number from event
    console.log(server.get('config').port); // also correct value
});

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[karol-f]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

[dignifiedquire]

Thanks, could you please

• add docs about this in https://github.com/karma-runner/karma/blob/master/docs/dev/04-public-api.md
• add a test to make sure this actually works as expected

[karol-f]

@dignifiedquire done :)

[dignifiedquire]

This is not really testing anything is it? You should probably emit the event on the mockWebServer instead

[karol-f]

I've changed test to pass when mockWebServer.listen callback is executed - so when server starts listening.

[dignifiedquire]

Thanks :) looks good now, if you could squash your commits into one then I can merge it.

[karol-f]

Done :)

[dignifiedquire]

Thanks Just waiting for travis to get green now

[karol-f]

@dignifiedquire It's green

[dignifiedquire]

@karol-f thanks :)

[segrey]

@karol-f thanks for the PR