Use the gametest framework and add a few tests

[altrisi]

Adds the framework to add gametests to Carpet Extra, and a few tests.

Now clean of hacks!

Useful to know for development commands and findings about them:

• Use /test export to export nearby structure blocks to test ready snbt files
  You don't even need to hit save on the block, it'll do it for you. Here's the different ones I've found useful:
  • exportclosest: The best, just exports the one form the structure block closest to you
  • exportthat: Exports "the one you're looking at". Sounds nice, but often doesn't work properly
  • export <name>: This one does need the structure to be saved I believe, and exports by name
  • exportthese: Exports all in quite a big radius. Don't use it, especially if you've ran tests given it'll run save on the modified blocks
• /test run of course
• /test verify seems to run them all in different places and orientations to ensure they work even if you move them
• /test pos to find the relative position inside a structure box (of the targeted block)
• tbh just check the /test command

Progress for #319, this PR won't complete it all.

[altrisi]

Was told the proper way, changed pretty much all of the framework I had done, and ended up removing all but a single line other than Gradle. Pushing changes in a bit.

[altrisi]

Will add it to GHA in later.