Expand most kotlinx.serialization tests on JS backend

to enhance and increase test coverage of the plugin.

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

@@ -1,4 +1,4 @@
-// TARGET_BACKEND: JVM_IR
+// IGNORE_BACKEND_K1: JS_IR
 
 // WITH_STDLIB
 
@@ -43,4 +43,4 @@ fun box(): String {
     val decodedEmail = Json.decodeFromString<Email<Int>>(encodedEmail)
     if (decodedEmail.toString() != "Email(foo, 1)") return "DecodedEmail: $decodedEmail"
     return "OK"
-}
+}