Run integration tests in-game

Name a more iconic duo than @SquidDev and over-engineered test
frameworks.

This uses Minecraft's test core[1] plus a home-grown framework to run
tests against computers in-world.

The general idea is:
 - Build a structure in game.
 - Save the structure to a file. This will be spawned in every time the
   test is run.
 - Write some code which asserts the structure behaves in a particular
   way. This is done in Kotlin (shock, horror), as coroutines give us a
   nice way to run asynchronous code while still running on the main
   thread.

As with all my testing efforts, I still haven't actually written any
tests!  It'd be good to go through some of the historic ones and write
some tests though. Turtle block placing and computer redstone
interactions are probably a good place to start.

[1]: https://www.youtube.com/watch?v=vXaWOJTCYNg

.gitattributes

@@ -1,2 +1,3 @@
 # Ignore changes in generated files
 src/generated/resources/data/** linguist-generated
+src/test/server-files/structures linguist-generated