[Serialization] Migrate all tests to new test infrastructure

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

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // TARGET_BACKEND: JVM_IR
 
 // WITH_STDLIB
@@ -19,4 +18,4 @@ fun box(): String {
     val decoded = Json.decodeFromString(Holder.serializer(), str)
     if (decoded.f1.t != holder.f1.t) return "f1.t: ${decoded.f1.t}"
     return "OK"
-}
+}