Bug report

This was initially reported as phpstan/phpstan-phpunit#118 but after digging into PHPStan code, I found out that certain rules make use of additionalConstructors parameter which is correctly set to PHPUnit\Framework\TestCase::setUp in that plugin.

I believe ReadOnlyPropertyAssignRule should respect additionalConstructors parameter to resolve all methods that are allowed to set readonly properties instead of hardcoding to __construct and __unserialize:
https://github.com/phpstan/phpstan-src/blob/4dd6b692ef9224725764a6daec1b680c9eb84e03/src/Rules/Properties/ReadOnlyPropertyAssignRule.php#L70

I noticed there are 2 other rules that already use this functionality:
https://github.com/phpstan/phpstan-src/blob/44fd938c30de5ee87f41b9c97b3d79ec18a731fa/src/Rules/Properties/MissingReadOnlyPropertyAssignRule.php
and
https://github.com/phpstan/phpstan-src/blob/28bd563fbafe2f4a884a6e77bba0149df98a8b93/src/Rules/Properties/UninitializedPropertyRule.php

but the code responsible for caching additionalConstructors is duplicated. Perhaps we could extract it into some kind of helper?

Code snippet that reproduces the problem

class SubjectTest extends \PHPUnit\Framework\TestCase
{
    protected readonly Subject $subject;

    protected function setUp(): void
    {
        parent::setUp();

        $this->subject = new Subject('test');
    }
}

with config:

parameters:
	additionalConstructors:
		- PHPUnit\Framework\TestCase::setUp

triggers: Readonly property App\Tests\SubjectTest::$subject is assigned outside of the constructor.

Runnable example: https://phpstan.org/r/974f222b-dcfa-470b-8281-a4e2dab6a954

Expected output

No errors should be produced.

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