[TASK] Add the first basic functional tests for Parser (#1020)

Part of #757

Author: oliverklee

tests/Functional/ParserTest.php

@@ -0,0 +1,39 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Sabberworm\CSS\Tests\Functional;
+
+use PHPUnit\Framework\TestCase;
+use Sabberworm\CSS\CSSList\Document;
+use Sabberworm\CSS\Parser;
+
+/**
+ * @covers \Sabberworm\CSS\Parser
+ */
+final class ParserTest extends TestCase
+{
+    /**
+     * @test
+     */
+    public function parseWithEmptyStringReturnsDocument(): void
+    {
+        $parser = new Parser('');
+
+        $result = $parser->parse();
+
+        self::assertInstanceOf(Document::class, $result);
+    }
+
+    /**
+     * @test
+     */
+    public function parseWithOneRuleSetReturnsDocument(): void
+    {
+        $parser = new Parser('.thing { }');
+
+        $result = $parser->parse();
+
+        self::assertInstanceOf(Document::class, $result);
+    }
+}