[Serialization] Migrate all tests to new test infrastructure

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

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // TARGET_BACKEND: JVM_IR
 
 // WITH_STDLIB
@@ -34,4 +33,4 @@ fun box(): String {
     desc.checkKind(1, "OPEN")
     desc.checkKind(2, "SEALED")
     return "OK"
-}
+}