Remove unnecessary custom tests/bootstrap.php PHPUnit bootstrap to load phpstan preload

[samsonasik]

I tested locally, even without custom tests/bootstrap.php bootstrap, phpunit seems working ok:

➜  phpstan-error-formatter git:(remove-unnessary-custom-bootstrap) vendor/bin/phpunit 
PHPUnit 9.6.21 by Sebastian Bergmann and contributors.

Runtime:       PHP 8.3.12
Configuration: /Users/samsonasik/www/phpstan-error-formatter/phpunit.xml

....................                                              20 / 20 (100%)

Time: 00:00.002, Memory: 8.00 MB

OK (20 tests, 20 assertions)

This PR remove it as seems not necessary.

[samsonasik]

Ready to merge 👍