test(getNextReleaseType): add tests for false breaking changes

[kettanaito]

This library has produced a number of false breaking changes in the past:

• https://github.com/ossjs/release/commit/0caf7abf74ce195e4bc1990311976910f9d7e21a, which triggered a major version bump while it shouldn't).
• https://github.com/mswjs/headers-polyfill/commit/dd0b62489786d0f9e44d36d570bebed8169512b9, where it should've triggered a major bump but didn't (admittedly, my mistake; should be BREAKING CHANGE: with the semicolon).

This PR improves this behavior.

It would be great to have a manual approval on any potential breaking change release from the developer. It seems that GitHub Actions don't support manual approvals amidst workflows. That's a shame.

There are some alternatives, like https://github.com/marketplace/actions/manual-workflow-approval, but they introduce significantly different release workflow than I'd prefer (no PRs/issues/etc).