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/src/main/kotlin/cc/tweaked/gradle/CCTweakedPlugin.kt

@@ -2,6 +2,7 @@ package cc.tweaked.gradle
 
 import org.gradle.api.Plugin
 import org.gradle.api.Project
+import org.gradle.jvm.toolchain.JavaLanguageVersion
 
 /**
  * Configures projects to match a shared configuration.
@@ -10,4 +11,8 @@ class CCTweakedPlugin : Plugin<Project> {
     override fun apply(project: Project) {
         project.extensions.create("cct", CCTweakedExtension::class.java)
     }
+
+    companion object {
+        val JAVA_VERSION = JavaLanguageVersion.of(8)
+    }
 }