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).

src/main/java/dan200/computercraft/core/computer/ComputerThread.java

@@ -5,6 +5,7 @@
  */
 package dan200.computercraft.core.computer;
 
+import com.google.common.annotations.VisibleForTesting;
 import com.google.errorprone.annotations.concurrent.GuardedBy;
 import dan200.computercraft.ComputerCraft;
 import dan200.computercraft.core.ComputerContext;
@@ -443,7 +444,8 @@ public final class ComputerThread
      *
      * @return If we have work queued up.
      */
-    boolean hasPendingWork()
+    @VisibleForTesting
+    public boolean hasPendingWork()
     {
         // FIXME: See comment in scaledPeriod. Again, we access this in multiple threads but not clear if it matters!
         return !computerQueue.isEmpty();