promise_test: add case for incorrect number of callbacks

Author: Rob Figueiredo

promise_test.go

@@ -104,3 +104,24 @@ func TestPromiseRejected(t *testing.T) {
 t.Fatalf("expected call to onRejected, got none")
 }
 }
+
+func TestPromiseThenPanic(t *testing.T) {
+t.Parallel()
+
+iso, _ := v8go.NewIsolate()
+ctx, _ := v8go.NewContext(iso)
+res, _ := v8go.NewPromiseResolver(ctx)
+prom := res.GetPromise()
+
+t.Run("no callbacks", func(t *testing.T) {
+defer func() { recover() }()
+prom.Then()
+t.Errorf("expected a panic")
+})
+t.Run("3 callbacks", func(t *testing.T) {
+defer func() { recover() }()
+fn := func(_ *v8go.FunctionCallbackInfo) *v8go.Value { return nil }
+prom.Then(fn, fn, fn)
+t.Errorf("expected a panic")
+})
+}