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.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);
+});