[K/N] Move special backend checks test sources to diagnostic tests folder

^KT-61564

compiler/testData/diagnostics/nativeTests/specialBackendChecks/objCInterop/t31.kt

@@ -0,0 +1,10 @@
+import kotlinx.cinterop.*
+import platform.darwin.*
+import platform.Foundation.*
+
+class Zzz : NSAssertionHandler() {
+    @ObjCOutlet
+    var x: String 
+        get() = "zzz"
+        set(value: String) { }
+}