Refactor deprecation warning tests

[distantnative]

PHPUnit 10 won't throw errors anymore, which is why we need to switch to a different test setup for our deprecation errors

[lukasbestle]

Looking into the CI failure. Something seems to be off as my commit worked locally before but now works neither in CI nor locally.

[lukasbestle]

@distantnative Reverted my earlier commit as it broke subsequent tests, but I couldn't find out why.

From reading the code, this test should not need to restore the error handler as the one set inside Helpers::handleErrors() is already reset within the method and no other error handler is set:

kirby/tests/Cms/Helpers/HelpersTest.php

Lines 198 to 207 in ba8e9e5

	public function testHandleErrorsWarningCaughtCallbackValue()
	{
	$this->hasErrorHandler = true;
	$this->assertSame('handled', Helpers::handleErrors(
	fn () => trigger_error('Some warning', E_USER_WARNING),
	fn (int $errno, string $errstr) => true,
	fn () => 'handled'
	));
	}

Can you remember why you added the hasErrorHandler line in this test?