This is coming from a cofounder at github, someone who probably knows precisely what the GPL is for. Whatever the legal merits, building on a GPL3 project's complete test suite and relicensing under MIT is not acting in good faith toward the original authors. I really find it disgusting and it makes me want to avoid gitbutler entirely.
I think you're saying that you don't believe in the freedoms to use the GPL licensed test suite for certain purposes which are explicitly allowed by the GPL.
You don't get to choose a license and then add extra terms to it when you don't feel like it's up to scratch. That's something explicitly not allowed by the GPL license.