Add CallableProvider trait for tests

src/Tests/Tools/CallableProvider.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace Toalett\Multiprocessing\Tests\Tools;
+
+trait CallableProvider
+{
+    protected static function emptyCallable(): callable
+    {
+        static $fn = null;
+        $fn ??= static fn() => null;
+        return $fn;
+    }
+}