Added debug message section.

[siad007]

This PR:

• Adds new debug message section to the DebugFileLogger
• Covered by tests
• Doc PR: https://github.com/infection/site/pull/XXX

Fixes #520

[maks-rafalko]

File DebugFileLogger is used for end-to-end tests only. It's not supposed to be used for developers.
We have recently removed it from the documentation, but it hasn't been released yet.

I've added a comment to this class that it's for e2e tests only.

[maks-rafalko]

I'm not sure MetricsCalculator should have debugMessage array, this looks a bit confusing.

Since the writing to log files is done through subscribers (see MutationTestingResultsLoggerSubscriber), I would suggest the following:

1. Add a initial tests run process to the InitialTestSuiteStarted() event's constructor here:

infection/src/Process/Runner/InitialTestsRunner.php

Line 74 in 096d7b2

$this->eventDispatcher->dispatch(new InitialTestSuiteStarted());

2. Subscribe to this InitialTestSuiteStarted event in MutationTestingResultsLoggerSubscriber here

infection/src/Process/Listener/MutationTestingResultsLoggerSubscriber.php

Lines 104 to 109 in 096d7b2

	public function getSubscribedEvents(): array
	{
	return [
	MutationTestingFinished::class => [$this, 'onMutationTestingFinished'],
	];
	}

3. Extract the process (or command line string) from the InitialTestSuiteStarted event and use it in MutationTestingResultsLoggerSubscriber::onMutationTestingFinished() (which is executed later than InitialTestSuiteStarted)

[maks-rafalko]

I think it would be convenient to add this debug info to infection.log when --debug option is used

Also, the command line should be logged only when --debug option is used, since we are trying to reduce the number of generated information from the log files by default.

[BackEndTea]

Instead of adding it to the infection.log, why not output it to the terminal (when using --debug of course)?

[maks-rafalko]

• We didn't output anything for debugging purposes before to the terminal
• Mutation command lines are logged to the infection.log, so adding initial tests run command line there seemed to me consistent.

However, I'm ok with adding it to stdout.

[maks-rafalko]

Closing in favor of #683. Thanks @siad007 for your efforts.