Removed unused files in tests

[franmomu]

I was trying to add missing types and bumped into these files that apparently are not used.

CallbackStrategyTestHelper was added in #871 and looks like it has been replaced by a callback:

Elastica/tests/Connection/Strategy/CallbackStrategyTest.php

Lines 25 to 33 in 8fe6176

	$callback = static function ($connections) use (&$count): Connection {
	++$count;
	return \current($connections);
	};
	$mock = $this->createMock(Connection::class);
	$strategy = new CallbackStrategy($callback);
	$strategy->getConnection([$mock]);

ErrorsCollector was added in #1031 and its usage was removed in #1742.

[ruflin]

Thx for the cleanup.