Add a first test POC

extra/html-extra/Tests/HtmlAttrMergeTest.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace Twig\Extra\Html\Tests;
+
+use PHPUnit\Framework\TestCase;
+use Twig\Extra\Html\HtmlExtension;
+
+class HtmlAttrMergeTest extends TestCase
+{
+    /**
+     * @dataProvider htmlAttrProvider
+     */
+    public function testMerge(array $expected, array $inputs): void
+    {
+        $result = HtmlExtension::htmlAttrMerge(...$inputs);
+
+        self::assertSame($expected, $result);
+    }
+
+    public function htmlAttrProvider(): \Generator
+    {
+        yield 'simple test' => [
+            ['id' => 'some-id', 'class' => ['some-class']],
+            [
+                ['id' => 'some-id'],
+                ['class' => 'some-class'],
+            ]
+        ];
+    }
+}