Add an initial test for counting past talks

tests/TalkExtensionTest.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace App\Tests;
+
+use App\TwigExtension\TalkExtension;
+use PHPUnit\Framework\TestCase;
+
+final class TalkExtensionTest extends TestCase
+{
+    /** @test */
+    public function it_returns_zero_if_there_are_no_talks(): void
+    {
+        $this->assertSame(0, (new TalkExtension())->getPastTalksCount([]));
+    }
+}