Share test fixtures across modules

- Move art.arcane.iris.testsupport into core/src/testFixtures and apply java-test-fixtures to core
- Consume core test fixtures from the bukkit plugin, nms, probe and the three loader adapters
- Delete the duplicate ProjectPaths copies in the bukkit-plugin and modded-common test trees
- Point ModdedMessageContributorTest at the shared ProjectPaths
- Guard CommandPackCompatTest, NativeTransitionColumnTest and StubPlatformStateTest with PlatformLeakGuard
- Release the probe runtime state after DimensionStackSuperflatProbeTest opens a workspace
- Keep core main classes ahead of the core jar on the test classpath so the bytecode gates still scan a directory

adapters/bukkit/nms/v26_2_R1/src/test/java/art/arcane/iris/nativegen/NativeTransitionColumnTest.java

@@ -6,6 +6,7 @@ import art.arcane.iris.spi.IrisPlatform;
 import art.arcane.iris.spi.IrisPlatforms;
 import art.arcane.iris.spi.PlatformBlockState;
 import art.arcane.iris.spi.PlatformRegistries;
+import art.arcane.iris.testsupport.PlatformLeakGuard;
 import net.minecraft.SharedConstants;
 import net.minecraft.core.Direction;
 import net.minecraft.server.Bootstrap;
@@ -16,6 +17,7 @@ import net.minecraft.world.level.block.RotatedPillarBlock;
 import net.minecraft.world.level.block.LiquidBlock;
 import net.minecraft.world.level.levelgen.Heightmap;
 import org.junit.BeforeClass;
+import org.junit.Rule;
 import org.junit.Test;
 import org.mockito.MockedStatic;
 
@@ -30,6 +32,9 @@ import static org.mockito.Mockito.mockStatic;
 import static org.mockito.Mockito.when;
 
 public final class NativeTransitionColumnTest {
+    @Rule
+    public final PlatformLeakGuard leakGuard = PlatformLeakGuard.clean();
+
     @BeforeClass
     public static void bootstrap() {
         SharedConstants.tryDetectVersion();