Add test for #7960 - custom autoloader is called twice

[mvorisek]

add test for #7960

the problem is present when:

• a class is autoloaded from phpstan
• and the class has a parent class
• and both classes are unloaded
• and both classes are loaded using dynamic autoloader

in such scenario, phpstan tries to call the autoloader twice, which is wrong as it should never be called more than one for one name

CI: https://github.com/phpstan/phpstan/actions/runs/4784807550/jobs/8506769532?pr=9232#step:7:24

(when if (class_exists($name)) { return; } is added to the custom autoloader, phpstan does finish as expected without an error)

[mvorisek]

@ondrejmirtes this repro/test sits here for a whole, would you be ok to look at it? Thank you.

[ondrejmirtes]

Thank you.