Suppress a Ruby warning when using Ruby 3.3.0dev

Follow up ruby/ruby@1c93288.

This PR suppresses the following warning when using Ruby 3.3.0dev

```consle
$ cd path/to/rubocop
$ ruby -v
ruby 3.3.0dev (2023-08-22T14:46:32Z master 7127f39bac) [x86_64-darwin22]
$ bundle exec rake spec
/Users/koic/.rbenv/versions/3.3.0-dev/lib/ruby/gems/3.3.0+0/gems/crack-0.4.5/lib/crack/xml.rb:9:
 warning: bigdecimal will be not part of the default gems since Ruby 3.4.0. Add it to your Gemfile.
```

crack is a dependency for the testing tool WebMock, so it will be added to the Gemfile:

```console
$ cat Gemfile.lock
(snip)
    webmock (3.18.1)
      addressable (>= 2.8.0)
      crack (>= 0.3.2)
      hashdiff (>= 0.4.0, < 2.0.0)
```

The `bigdecimal` dependency can be removed when jnunemaker/crack#75 is merged and released.
It's a workaround until then.

Gemfile

@@ -22,6 +22,9 @@ gem 'test-queue'
 gem 'yard', '~> 0.9'
 
 group :test do
+  # FIXME: This `bigdecimal` dependency can be removed when https://github.com/jnunemaker/crack/pull/75
+  # is merged and released. It's a workaround until then.
+  gem 'bigdecimal', platform: :mri
   gem 'webmock', require: false
 end