Expand most kotlinx.serialization tests on JS backend

to enhance and increase test coverage of the plugin.

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

@@ -1,5 +1,3 @@
-// TARGET_BACKEND: JVM_IR
-
 // WITH_STDLIB
 
 import kotlinx.serialization.*
@@ -34,4 +32,4 @@ fun box(): String {
     val boxDesc = Box.serializer(String.serializer()).descriptor
     if (boxDesc.toString() != "Box(boxed: kotlin.String)") return boxDesc.toString()
     return "OK"
-}
+}