Expand most kotlinx.serialization tests on JS backend

to enhance and increase test coverage of the plugin.

plugins/kotlinx-serialization/testData/boxIr/multimoduleInheritanceJs.kt

@@ -0,0 +1,89 @@
+// WITH_STDLIB
+// TARGET_BACKEND: JS_IR
+// TARGET_BACKEND: JS_IR_ES6
+
+// JS vs JVM difference is that we are not able to understand that we have a default value in other module (KT-62523)
+
+// MODULE: lib
+// FILE: lib.kt
+
+package a
+
+import kotlinx.serialization.*
+
+@Serializable
+open class OpenBody {
+    var optional: String? = "foo"
+}
+
+@Serializable
+abstract class AbstractConstructor(var optional: String = "foo")
+
+@Serializable
+open class Vehicle {
+    var color: String? = null
+    var name: String? = null
+}
+
+
+// MODULE: app(lib)
+// FILE: app.kt
+
+package test
+
+import a.*
+import kotlinx.serialization.*
+import kotlinx.serialization.json.*
+import kotlin.test.assertEquals
+
+@Serializable
+class Test1: OpenBody()
+
+@Serializable
+class Test2: AbstractConstructor()
+
+@Serializable
+open class Car : Vehicle() {
+    var maxSpeed: Int = 100
+}
+
+fun test1() {
+    val string = Json.encodeToString(Test1.serializer(), Test1())
+    assertEquals("{\"optional\":\"foo\"}", string)
+    val reconstructed = Json.decodeFromString(Test1.serializer(), string)
+    assertEquals("foo", reconstructed.optional)
+}
+
+fun test2() {
+    val string = Json.encodeToString(Test2.serializer(), Test2())
+    assertEquals("{\"optional\":\"foo\"}", string)
+    val reconstructed = Json.decodeFromString(Test2.serializer(), string)
+    assertEquals("foo", reconstructed.optional)
+}
+
+fun test3() {
+    val json = Json { allowStructuredMapKeys = true; encodeDefaults = true }
+
+    val car = Car()
+    car.maxSpeed = 100
+    car.name = "ford"
+    val s = json.encodeToString(Car.serializer(), car)
+    assertEquals("""{"color":null,"name":"ford","maxSpeed":100}""", s)
+    val restoredCar = json.decodeFromString(Car.serializer(), s)
+    assertEquals(100, restoredCar.maxSpeed)
+    assertEquals("ford", restoredCar.name)
+    assertEquals(null, restoredCar.color)
+}
+
+fun box(): String {
+    try {
+        test1()
+        test2()
+        test3()
+        return "OK"
+    } catch (e: Throwable) {
+        e.printStackTrace()
+        return e.message!!
+    }
+
+}