Expand most kotlinx.serialization tests on JS backend

to enhance and increase test coverage of the plugin.

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

@@ -1,5 +1,3 @@
-// TARGET_BACKEND: JVM_IR
-
 // WITH_STDLIB
 
 // FILE: a.kt
@@ -45,4 +43,4 @@ class Holder(
 fun box(): String {
     val d = Holder.serializer().descriptor.toString()
     return if (d == "a.Holder(i: kotlin.Int, c: MySer<kotlin.Int>)") "OK" else d
-}
+}