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

^KT-61564

compiler/testData/diagnostics/nativeTests/specialBackendChecks/concurrent/t51.kt

@@ -0,0 +1,11 @@
+@file:OptIn(ObsoleteWorkersApi::class)
+import kotlin.native.concurrent.*
+
+class Z(val x: Int) {
+    fun bar(s: String) = s + x.toString()
+}
+
+class Q(x: Int) {
+    val z = Worker.start().execute(TransferMode.SAFE, { "zzz" }, Z(x)::bar)
+}
+