Run garbage collector on AfterLastTestMethodFinished

[rr-it]

The garbage collector can be already configured to run after every X (default: 100) Tests.

I realized most memory can be freed up when a test-case with all its tests is finished:

May we add a configuration option to also trigger the garbage collector on event AfterLastTestMethodFinished?

E.g. trigger a new function GarbageCollectionHandler::lastTestMethodFinished

    public function lastTestMethodFinished(): void
    {
            gc_collect_cycles();

            $this->facade->emitter()->testRunnerTriggeredGarbageCollection();

            $this->tests = 0;
    }

Current triggers:

phpunit/src/Runner/GarbageCollection/GarbageCollectionHandler.php

Lines 75 to 86 in 3e6c0dc

	/**
	* @throws EventFacadeIsSealedException
	* @throws UnknownSubscriberTypeException
	*/
	private function registerSubscribers(): void
	{
	$this->facade->registerSubscribers(
	new ExecutionStartedSubscriber($this),
	new ExecutionFinishedSubscriber($this),
	new TestFinishedSubscriber($this),
	);
	}

[rr-it]

This is related to #4705

If the properties of a test-object are unset in tearDown then memory must still be cleaned up via garbage collector eg. in tearDownAfterClass method = event AfterLastTestMethodFinished.

[rr-it]

This is not necessary anymore as the memory issue is resolved by:
#5861