Argument 2 passed to Drupal\Console\Command\Test\RunCommand::__construct() must be an instance of Drupal\simpletest\TestDiscovery
This is really just kiamlaluno's answer, but they never posted it so I'll just copy it here verbatim:

Drupal Console has not been updated to run on new Drupal versions.
It's even less compatible with Drupal 9, as many classes or methods
have been removed.