Use symfony/phpunit-bridge

[Nyholm]

This might seam unnecessary. However, I believe it will be a great additions.

1. The tests in Added helper class for extension #162 is failing on php 7.1 because the PHPUnit 7.5 does not include some methods. We worked our way around this by implementing an abstract TestCase ourselves.
2. Our abstract TestCase always loads (soon to be legacy) providers and instantiating a new Generator. That is not required for most tests. See Added helper class for extension #162, [Modifiers] Helper methods to use our custom generators #155 and Adding support for extensions and PSR-11 #154
3. We will soon start to deprecate methods. The symfony/phpunit-bridge will make sure we are not using deprecated code internally.
4. We can probably start to use features in PHPUnit 10 that will be released in a few months.

And also, this is a dev dependency, so it does not really impact the project negatively.

[Nyholm]

Thank you for merging