Raise an exception if tests or changelog generation fails

[G-Rath]

I realised while working on my recent PRs that CI wasn't actually failing when bundle exec rake [test] reported errors - turns out that tests are being invoked by system which by default does not do any error handling; instead it sets $? to the exit code which can then be checked.

Ruby 2.6 added an exception option which when true causes system to raise an exception if the command exits with a non-zero code, making this very easy to address.

I've checked the codebase and confirmed these are the only uses of system.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• 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.

[G-Rath]

Ok so it looks like the JRuby and "oldest supported RuboCop version" jobs are already failing which this change has revealed - I've added an env variable to control if test failures should actually be surfaced to use in those two jobs so that this PR can be landed as a dedicated change, and then those jobs can be addressed in their own dedicated PRs (since "mostly working CI" is better than "not working CI" and I have no context on either of the failures so expect they'll take more time to resolve).

I've kept this as a separate commit in case you want to do something different @koic, but if you're happy with this path I can squash it.

Also it looks like MultipleAssertionsTest#test_generates_a_todo_based_on_the_worst_violation is flakey - I've confirmed this locally: running multiple times, every so often I get a failure with an incorrect todo count; again something to resolve in a dedicated PR - for now just rerunning that job for this PR should get it passing.

[koic]

Is there any problem with always setting it to true for exception option?

Suggested change

	error_on_failure = ENV.fetch('ERROR_ON_TEST_FAILURE', 'true') != 'false'
	system("bundle exec minitest-queue #{Dir.glob('test/**/*_test.rb').shelljoin}", exception: error_on_failure)
	system("bundle exec minitest-queue #{Dir.glob('test/**/*_test.rb').shelljoin}", exception: true)

[G-Rath]

Yes, two of the jobs are currently failing and I think fixing them should be done in dedicated PRs - once they're fixed, this can then be set to true.

[koic]

Ah, OK. Let's merge this first 👍