Improve checkbinary to attempt to find heroic binary on PATH

[afwolfe]

Hello, thanks for this great tool. I've been using it for awhile now on my Fedora Linux system to quickly launch games from Steam while using the RPM-packaged version of Heroic Games Launcher from the recommended COPR repo (atim/heroic-games-launcher).

Unfortunately, due to the hard-coding of the expected paths of Heroic in HeroicBashLauncher, it wasn't working on my system at first because the RPM version of Heroic is installed to /usr/lib64/heroic-games-launcher-bin/.

This PR attempts to fix this issue and allow for more "diverse" Heroic installations in the future by:

• Attempting to find heroic on the PATH before going back to the old defaulting logic.
• Including unit tests for checkbinary.py to confirm that each of the correct paths are returned in a few different scenarios.
  • These can be invoked with python -m unittest tests/*.py
  • A side-effect of making the modules testable required reorganizing the main HeroicBashLauncher.py to nest its code under an if __name__ == "__main__": block and converting a few of the checks in settings.py to be functions that are now called from this main method.

[redromnon]

Thanks a lot for this @afwolfe!

[afwolfe]

Hello @redromnon, I reverted the changes we discussed for now and this is ready for a (hopefully) final review. Please take your time, and thanks again for your help and feedback.

I'll open another PR for the Flatpak detection fix once this is merged so I can build off of these changes.