add singleton trait test

tests/ZM/Utils/SingletonTraitTest.php

@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tests\ZM\Utils;
+
+use ZM\Utils\SingletonTrait;
+use PHPUnit\Framework\TestCase;
+
+class SingletonTraitTest extends TestCase
+{
+    public function testGetInstance(): void
+    {
+        $mock = $this->getObjectForTrait(SingletonTrait::class);
+        $this->assertEquals($mock, $mock::getInstance());
+    }
+}