Clean up new system tests

PR https://github.com/googleapis/google-cloud-php/pull/7695 introduced system tests as part of our workflow, but there's still a lot of work to be done.

- As mentioned in https://github.com/googleapis/google-cloud-php/pull/7695#discussion_r1778955348, we should keep all ENV vars in one place and define them as constants
- Remove `tests/System/bootstrap.php` files if possible, or consolidate them to use one in `Core`
- Cleanup use of `PROJECT_ID` environment variable, in favor of `GOOGLE_PROJECT_ID`, `GOOGLE_CLOUD_PROJECT`, or remove it all together
- Cleanup use of `TestHelpers` to make all tests consistent.