Set group only if we don't do add logic

Setting the new group before the add logic causes breaks the
removal from the thread's old group. Instead, only directly set
the thread's group if we won't run the add logic.

core/src/main/java/org/jruby/RubyThreadGroup.java

@@ -95,12 +95,11 @@ public IRubyObject add(ThreadContext context, IRubyObject rubyThread, Block bloc
             }
         }
 
-        // always set group when added explicitly
-        thread.setThreadGroup(this);
-
         // we only add live threads
         if (thread.alive_p(context).isTrue()) {
             addDirectly(thread);
+        } else {
+            thread.setThreadGroup(this);
         }
 
         return this;