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

^KT-61564

compiler/testData/diagnostics/nativeTests/specialBackendChecks/objCInterop/t2.kt

@@ -0,0 +1,6 @@
+import platform.darwin.*
+import platform.Foundation.*
+
+class Zzz : NSAssertionHandler() {
+    override fun handleFailureInFunction(functionName: String, file: String, lineNumber: NSInteger /* = Long */, description: String?, vararg args: Any?) { }
+}