A PHPUnit 10 compatible version of the PHPUnit Polyfills has been released, so let's start using it.

Ref:
* https://github.com/Yoast/PHPUnit-Polyfills/releases/tag/2.0.0

As the available traits are different between the 1.x and 2.x versions of the PHPUnit Polyfills, two different class definitions are needed.

This change uses the available PHPUnit Polyfills version number to load the correct class.

As of PHPUnit 10, data providers are (again) expected to be `static` methods.

This updates the test suite to respect that.

Includes removing the use of `$this` from select data providers.

Refs:
* sebastianbergmann/phpunit@9caafe2
* sebastianbergmann/phpunit 5100

…h PHPUnit 10

…patibility with PHPUnit 10

…se::getName()` method

... in favour of just using a randomly generated ID for the cache.

…back()` method

PHPUnit 10.0 removed the `TestCase::setOutputCallback()` method without replacement.

Refs:
* sebastianbergmann/phpunit 5319

... to provide a work-around for the removal of the `InvocationMocker->withConsecutive()` method, which was removed without replacement in PHPUnit 10.0.

Refs:
* sebastianbergmann/phpunit 4026
* sebastianbergmann/phpunit 4255
* sebastianbergmann/phpunit 4564

…WithConsecutiveArgs` helper

…::setMethods()` removal

Please note:
* The `getMockBuilder()` class is also deprecated now, so replacing with another method call on the `MockBuilder` class will only work in the short/medium term and in a future iteration, this will have to be refactored again.
* Also the `getMockForAbstractClass method is also deprecated and expected to be removed in PHPUnit 12.

Refs:
* sebastianbergmann/phpunit 3687#issuecomment-492537584
* sebastianbergmann/phpunit 3770
* sebastianbergmann/phpunit 3769
* sebastianbergmann/phpunit 4775

PHPUnit 10 makes significant changes to the configuration file.
While there is a `--migrate-configuration` option available in PHPUnit, that doesn't get us a well enough converted configuration file, so instead use a separate `phpunit10.xml.dist` file for running the tests on PHPUnit 10 with an optimal configuration.

Includes
* Adding separate scripts to the `composer.json` file to make this more obvious for contributors.
* Updating the GH Actions workflows to take the new configuration file into account when appropriate.
* Selectively not using the `--repeat` option, which was [removed without replacement](sebastianbergmann/phpunit#5174) in PHPUnit 10.