[Serialization] Migrate all tests to new test infrastructure

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

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // TARGET_BACKEND: JVM_IR
 
 // WITH_STDLIB
@@ -65,4 +64,4 @@ fun box(): String {
     assertEquals("ObjectSerializerDummy", descs[3].serialName)
     assertEquals("ClassSerializerOnClass", descs[4].serialName)
     return "OK"
-}
+}