Make live tests required PR checks

[shenglol]

Currently the live tests are optional checks, and they are skipped for PRs sent from forks. However, failures in the live tests are usually ignored blindly as the test credentials expire quite often. We should consider making them required checks instead to force people to go through the error messages. This can help prevent regressions like #6823.

One thing we need to figure out is how to make the checks pass silently for PRs sent from forks, since required checks cannot be skipped (https://github.community/t/feature-request-conditional-required-checks/16761).