Tests should confirm that `Login with OpenAppStack` OIDC button exists

I know for certain that the Nextcloud test doesn't do this at the moment. Also check the other tests.

Our current test don't use the single sign-on provider to log in, because we don't have valid TLS certificates in the CI environment. However, we can make sure that SSO is at least somewhat configured by confirming that there is a button to sign in with OIDC on the login page.

This should help us flag issues like https://open.greenhost.net/openappstack/nextcloud/-/issues/41 as soon as they arise.