* Add `make test-bundler-parallel` executed on the upstream CI.
See <41a48535d1/.github/workflows/ubuntu.yml (L22)>.
* The tests are disabled as a default, as it requires the internet.
Run the Bundler tests as follows.
```
$ mock --with bundler_tests ruby-3.1.0-161.fc37.src.rpm
```
* Fix a test for `bin/bundle update --bundler` in `make test-bundler`.
