Depend on core metapackages for forward compatibility
Right now, this package is not compatible with Drupal 9, because Drupal 9 requires PHPUnit 8. We should at least declare compatibility with PHPUnit 8 so that this can be installed into a Drupal 9 project, and make any other changes required for PHPUnit 8 compatibility.
My own local testing confirms that it works just by adding an allowance to composer.json for PHPUnit 8 -- the TestCase.php shim that Drupal core uses to get around the type hinting added in PHPUnit 8 is unaffected by DTT.
After discussion with @weitzman, this merge request unbounds the phpunit/phpunit dependency so that consuming packages can install it into a Drupal 9 project. For internal testing, we use the drupal/core-dev metapackage to constrain things further. This, I believe, is the minimum set of changes we can make which will allow this to be used with D9.