Fix extension using incorrect name

Author: Jonathing

src/main/groovy/net/minecraftforge/testing/aggregate/AggregateTestPlugin.java

@@ -20,6 +20,6 @@ public AggregateTestPlugin() {
 
     @Override
     public void setup(Project project) {
-        project.getExtensions().create(NAME, AggregateTestExtensionImpl.class);
+        project.getExtensions().create(AggregateTestExtension.NAME, AggregateTestExtensionImpl.class);
     }
 }