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")
+}

testdata/wasmfunc.js

@@ -0,0 +1,21 @@
+require('../targets/wasm_exec.js');
+
+var callback;
+
+global.setCallback = (cb) => {
+    callback = cb;
+};
+
+global.callCallback = () => {
+    console.log('calling callback!');
+    let result = callback(1, 2, 3, 4);
+    console.log('result from callback:', result);
+};
+
+let go = new Go();
+WebAssembly.instantiate(fs.readFileSync(process.argv[2]), go.importObject).then((result) => {
+    go.run(result.instance);
+}).catch((err) => {
+    console.error(err);
+    process.exit(1);
+});

testdata/wasmfunc.txt

@@ -0,0 +1,7 @@
+calling callback!
+inside callback! parameters:
+  parameter: 1
+  parameter: 2
+  parameter: 3
+  parameter: 4
+result from callback: 10