add root fiber resume test to know double resume checking is working

mrbgems/mruby-fiber/test/fiber.rb

@@ -105,3 +105,13 @@ assert('Recursive resume of Fiber') do
   assert_false f2.alive?
   assert_false f3.alive?
 end
+
+assert('Root fiber resume') do
+  root = Fiber.current
+  assert_raise(RuntimeError) { root.resume }
+  f = Fiber.new {
+    assert_raise(RuntimeError) { root.resume }
+  }
+  f.resume
+  assert_false f.alive?
+end