Move some test support code into testFixtues

This offers very few advantages now, but helps support the following in
the future:

 - Reuse test support code across multiple projects (useful for
   multi-loader).
 - Allow using test fixture code in testMod. We've got a version of our
   gametest which use Kotlin instead of Lua for asserting computer
   behaviour.

We can't use java-test-fixtures here for Forge reasons, so have to roll
our own version. Alas.

 - Add an ILuaMachine implementation which runs Kotlin coroutines
   instead. We can use this for testing asynchronous APIs. This also
   replaces the FakeComputerManager.

 - Move most things in the .support module to .test.core. We need to use
   a separate package in order to cope with Java 9 modules (again,
   thanks Forge).

buildSrc/build.gradle.kts

@@ -9,6 +9,7 @@ repositories {
 }
 
 dependencies {
+    implementation(libs.kotlin.plugin)
     implementation(libs.spotless)
 }