[Serialization] Suppress OPT_IN errors in all diagnostic tests

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

@@ -1,5 +1,4 @@
 // FIR_IDENTICAL
-// !DIAGNOSTICS: -UNUSED_PARAMETER,-UNUSED_VARIABLE
 // WITH_STDLIB
 // SKIP_TXT
 // !USE_EXPERIMENTAL: kotlinx.serialization.ExperimentalSerializationApi
@@ -47,4 +46,4 @@ open class B2: A()
 @Serializable
 //@I("b")
 //@I2(E.A, E::class)
-open class C: B()
+open class C: B()