Improve Makefile testing

If there's a better way to exit with an error please tell me, I couldn't find anything through google.

This adds several changes to how the Makefile handles testing the test apps we have in the repo:

• Make the apps more consistent, just for me
• Add a clean-test target, for ease of cleaning up after running tests.
• Add the additional files to .gitignore, just in case

Closes #378 (closed) Closes #528 (closed)

EDIT: This MR used to add some conveniences for testing on non-standard branch names, however the branch exported is now stored only in project.conf, since we export directly to a flatpak repo. BuildStream only permits options with finite possibilities, so custom branch names are no longer feasible. I did consider adding a test branch name, but then realised this would require various elements to rebuild rather than be cached, and it didn't seem worth the cost.