[Wasm] Add JS interop tests

compiler/testData/codegen/boxWasmJsInterop/jsExport.kt

@@ -0,0 +1,19 @@
+// MODULE: main
+// FILE: externals.kt
+
+class C(val x: Int)
+
+@JsExport
+fun makeC(x: Int): C = C(x)
+
+@JsExport
+fun getX(c: C): Int = c.x
+
+fun box(): String = "OK"
+
+// FILE: jsExport__after.js
+
+const c = main.makeC(300);
+if (main.getX(c) !== 300) {
+    throw "Fail 1";
+}