Add some component level tests

tests/src/Components/Inlines/PlainTextTest.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace Erusev\Parsedown\Tests\Components\Inlines;
+
+use Erusev\Parsedown\Components\Inlines\PlainText;
+use Erusev\Parsedown\Parsing\Excerpt;
+use PHPUnit\Framework\TestCase;
+
+final class PlainTextTest extends TestCase
+{
+    /**
+     * @return void
+     * @throws \PHPUnit\Framework\ExpectationFailedException
+     * @throws \SebastianBergmann\RecursionContext\InvalidArgumentException
+     */
+    public function testPlainTextText()
+    {
+        $Plaintext = Plaintext::build(new Excerpt('foo', 0));
+
+        $this->assertSame('foo', $Plaintext->text());
+    }
+}