Add Bundler tests.
    
    * Add `make test-bundler-parallel` executed on the upstream CI.
      See <https://github.com/ruby/ruby/blob/41a48535d1992a08b5b7e9fdb5ac0dcb488bd724/.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`.