[Serialization] Suppress OPT_IN errors in all diagnostic tests

plugins/kotlinx-serialization/testData/diagnostics/NonSerializable.kt

@@ -1,4 +1,3 @@
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // FILE: test.kt
 import kotlinx.serialization.*
@@ -14,4 +13,4 @@ class Inside(val foo: List<<!SERIALIZER_NOT_FOUND("NonSerializable")!>NonSeriali
 @Serializable
 class WithImplicitType {
     <!SERIALIZER_NOT_FOUND("NonSerializable")!>val foo = NonSerializable()<!>
-}
+}