Create filter unit tests

Create test cases in the fashion of FilterUnitTestCase in the Filter.module test suite.

Test Cases

<h1>Heading Level 1</h1>
<h2>Heading Level 2</h2>
<h1>Heading Level 1</h1>Lorem ipsum
Lorem ipsum<h1>Heading Level 1</h1>
<h1><h2>Heading Level 1</h2></h1>
<h1>Heading Level 1</h1></h1>
<h1>Heading Level 1</h1><h2>Heading Level 2</h2>
<h1>Heading Level 1</h1><h2>Heading Level 2
<h1 id="level1">Heading Level 1</h1>
<h1 class="section">Heading Level 1</h1>