Add test case for this bug

- Fix whitespaces in MarkdownTest

tests/datasets/content/text/html/bug-7457.html

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+<h2 data-sourcepos="1:1-1:23" dir="auto">&#x000A;<a id="user-content-104-2019-08-01" class="anchor" href="#104-2019-08-01" aria-hidden="true"></a>[1.0.4] - 2019-08-01</h2>&#x000A;<h3 data-sourcepos="2:1-2:9" dir="auto">&#x000A;<a id="user-content-fixed" class="anchor" href="#fixed" aria-hidden="true"></a>Fixed</h3>&#x000A;<ul data-sourcepos="3:1-3:116" dir="auto">&#x000A;<li data-sourcepos="3:1-3:116">Invalid SemVer version generation, when the current branch does not have commits ahead of tag/checked out on a tag</li>&#x000A;</ul>

tests/datasets/content/text/html/bug-7457.txt

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+*[1.0.4] - 2019-08-01*
+
+*Fixed*
+
+* Invalid SemVer version generation, when the current branch does not have commits ahead of tag/checked out on a tag

tests/src/Content/Text/HTMLTest.php

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+<?php
+
+namespace Friendica\Test\src\Content\Text;
+
+use Friendica\Content\Text\HTML;
+use Friendica\Test\MockedTest;
+use Friendica\Test\Util\AppMockTrait;
+use Friendica\Test\Util\VFSTrait;
+
+class HTMLTest extends MockedTest
+{
+	use VFSTrait;
+	use AppMockTrait;
+
+	protected function setUp()
+	{
+		parent::setUp();
+		$this->setUpVfsDir();
+		$this->mockApp($this->root);
+	}
+
+	public function dataHTML()
+	{
+		$inputFiles = glob(__DIR__ . '/../../../datasets/content/text/html/*.html');
+
+		$data = [];
+
+		foreach ($inputFiles as $file) {
+			$data[str_replace('.html', '', $file)] = [
+				'input'    => file_get_contents($file),
+				'expected' => file_get_contents(str_replace('.html', '.txt', $file))
+			];
+		}
+
+		return $data;
+	}
+
+	/**
+	 * Test convert different input Markdown text into HTML
+	 *
+	 * @dataProvider dataHTML
+	 *
+	 * @param string $input    The Markdown text to test
+	 * @param string $expected The expected HTML output
+	 * @throws \Exception
+	 */
+	public function testToPlaintext($input, $expected)
+	{
+		$output = HTML::toPlaintext($input, 0);
+
+		$this->assertEquals($expected, $output);
+	}
+}