Minitest exit with non-zero status if coverage bellow minimum

[everton]

Fix #32

The minitest integration was exiting with success even if missing coverage. Ideally it should exit with an error status, so integration processes can be halted and failures easily detected.

I was not able to find a proper way to specify a minimum coverage, so I used the ENV var MINIMUM_COVERAGE (default 100%).

Types of Changes

• New feature.

Contribution

• [x] I added tests for my changes.
• [x] I tested my changes locally.
• [x] I agree to the Developer's Certificate of Origin 1.1.