Update test suite to phpunit 9.5 syntax
Refactored to use polyfill for older PHP versions via `yoast/phpunit-polyfills`
Note: this includes 2 important differences from usual phpunit test suites:
- instead of extending `PHPUnit\Framework\TestCase` we extend `Yoast\PHPUnitPolyfills\TestCases\TestCase`
- instead of handling fixtures via `setUp()` and `tearDown()` we use `set_up()` and `tear_down()` respectively
Comment regarding formatting: I chose to use the FQDN in the `extends` syntax of the class declaration instead of using `use` so that it is more quickly apparent that we're doing something slightly different than usual phpunit syntax, particularly in regards to the set_up() / tear_down() methods that appear immediately following the `extends` line.
