Expand most kotlinx.serialization tests on JS backend

to enhance and increase test coverage of the plugin.

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

@@ -1,5 +1,3 @@
-// TARGET_BACKEND: JVM_IR
-
 // WITH_STDLIB
 
 import kotlinx.serialization.*
@@ -20,4 +18,4 @@ fun box(): String {
     if (d.size != 1) return "Delegation to Map failed"
     if (d.data != dto.data) return "Equals failed ${d.data}"
     return "OK"
-}
+}