Run tests on GitHub actions

[tvdeyen]

Summary

• [x] Cache apt packages
• [x] Parallelize test runs
• [ ] Report test coverage

Checklist

Check out our PR guidelines for more details.

The following are mandatory for all PRs:

• [x] I agree that my PR will be published under the same license as Solidus.
• [x] I have written a thorough PR description.
• [x] I have kept my commits small and atomic.
• [x] I have used clear, explanatory commit messages.

The following are not always needed:

• 📖 I have updated the README to account for my changes.
• 📑 I have documented new code with YARD.
• 🛣️ I have opened a PR to update the guides.
• ✅ I have added automated tests to cover my changes.
• 📸 I have attached screenshots to demo visual changes.