Before #1227, end2end tests used to be run with WP-CLI or Selenium workers by switching a value in test-config.yml:

# Worker type to run the tests, 'selenium' or 'wp-cli'
end2end-test-type: selenium

IMO, this was always a bit of a hack as test-config was created to support various local testing environments, not to parameterize test runs – for that, we have TestRunnerOptions.php.

I see two options how to resolve this:

1. Move end2end-test-type to test runner options (and possibly rename it to end2end-workers or just workers).
2. Create two sets of test classes, one that runs with Selenium and one with WP-CLI, and get rid of runtime switching entirely.

I'd probably prefer the latter option as npm run tests should IMO truly run all the tests, not "either or".