[Serialization] Migrate all tests to new test infrastructure

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

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND_FIR: JVM_IR
 // TARGET_BACKEND: JVM_IR
 
 // WITH_STDLIB
@@ -45,4 +44,4 @@ fun testSerialization(previous: String): String {
 
 fun box(): String {
     return testSerialization(testSerializers())
-}
+}