wasm: add test for js.FuncOf

While there are some browser tests, Node.js is just a lot better for
testing this kind of stuff because it's much faster and we don't need a
browser for this.

testdata/wasmfunc.go

@@ -0,0 +1,17 @@
+package main
+
+import "syscall/js"
+
+func main() {
+	js.Global().Call("setCallback", js.FuncOf(func(this js.Value, args []js.Value) any {
+		println("inside callback! parameters:")
+		sum := 0
+		for _, value := range args {
+			n := value.Int()
+			println("  parameter:", n)
+			sum += n
+		}
+		return sum
+	}))
+	js.Global().Call("callCallback")
+}