[Wasm] Add JS interop tests

compiler/testData/codegen/boxWasmJsInterop/externals.kt

@@ -0,0 +1,25 @@
+// FILE: externals.js
+function createObject() {
+    return {};
+}
+
+function setX(obj, x) {
+    obj.x = x;
+}
+
+function getX(obj) {
+    return obj.x;
+}
+
+// FILE: externals.kt
+external interface Obj
+external fun createObject(): Obj
+external fun setX(obj: Obj, x: Int)
+external fun getX(obj: Obj): Int
+
+fun box(): String {
+    val obj = createObject()
+    setX(obj, 100)
+    if (getX(obj) != 100) return "Fail 2"
+    return "OK"
+}