Don't use `test-queue` to run tests on JRuby because it doesn't support `fork`

[G-Rath]

Follow up to #294

Currently this is aimed at being the most straightforward if probably slightly inelegant way to get tests running on JRuby - I assume there is probably a slightly cleaner way of invoking tests but I don't yet know what it is and Rake::TestTask was more than a few lines of code 😅

I think it would be worth considering having test-queue "support" JRuby by not using fork if that platform is detected or if you set TEST_QUEUE_WORKERS to 1 (or maybe 0?) as a way of making it easier to have a normalized way of invoking tests across different platforms like this; but it would also be equally valid I think for test-queue to say that's out of scope for the gem🤷

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Before submitting the PR make sure the following are checked:

• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• [x] Wrote good commit messages.
• [x] Commit message starts with [Fix #issue-number] (if the related issue exists).
• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [x] Added tests.
• [x] Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• [x] ~Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.~

[koic]

Thanks!