Expand most kotlinx.serialization tests on JS backend

to enhance and increase test coverage of the plugin.

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

@@ -1,5 +1,3 @@
-// TARGET_BACKEND: JVM_IR
-
 // WITH_STDLIB
 
 import kotlinx.serialization.*
@@ -13,4 +11,4 @@ fun box(): String {
     val kind = Ref.serializer().descriptor.kind.toString()
     if (kind != "CONTEXTUAL") return kind
     return "OK"
-}
+}