test(heading): add test for aria-labels being passed properly (#234)

[<img height=20 src=https://readme.com/static/favicon.ico align=center>][demo] | Fixes RM-1571
:---:|:---:

## 🧰 Changes

- Test if aria-label is being applied correctly — Related to this [PR](#230)

## 🧬 QA & Testing

Does the snapshot of the heading component look as expected?

- [Broken on production][prod].
- [Working in this PR app][demo].

[demo]: https://markdown-pr-234.herokuapp.com
[prod]: https://developers.readme.io

__tests__/__snapshots__/index.test.js.snap

@@ -1002,6 +1002,8 @@ exports[`export multiple Markdown renderers renders plain markdown as React 1`]
 
 exports[`export multiple Markdown renderers renders plainText from AST 1`] = `"Hello World"`;
 
+exports[`heading 1`] = `"<h2 class=\\"heading heading-2 header-scroll\\" align=\\"\\"><div class=\\"heading-anchor anchor waypoint\\" id=\\"example-header\\"></div><div class=\\"heading-text\\"><div id=\\"section-example-header\\" class=\\"heading-anchor_backwardsCompatibility\\"></div>Example Header</div><a aria-label=\\"Skip link to Example Header\\" class=\\"heading-anchor-icon fa fa-anchor\\" href=\\"#example-header\\"></a></h2>"`;
+
 exports[`image 1`] = `"<p><span aria-label=\\"Image\\" class=\\"img\\" role=\\"button\\" tabindex=\\"0\\"><img src=\\"http://example.com/image.png\\" alt=\\"Image\\" align=\\"\\" caption=\\"\\" height=\\"auto\\" title=\\"\\" width=\\"auto\\"><span class=\\"lightbox\\" role=\\"dialog\\" tabindex=\\"0\\"><span class=\\"lightbox-inner\\"><img src=\\"http://example.com/image.png\\" align=\\"\\" caption=\\"\\" height=\\"auto\\" title=\\"Click to close...\\" width=\\"auto\\" alt=\\"Image\\" class=\\"lightbox-img\\" loading=\\"lazy\\"></span></span></span></p>"`;
 
 exports[`list items 1`] = `

__tests__/index.test.js

@@ -34,6 +34,10 @@ test('image', () => {
   expect(mount(markdown.default('![Image](http://example.com/image.png)')).html()).toMatchSnapshot();
 });
 
+test('heading', () => {
+  expect(mount(markdown.default('## Example Header')).html()).toMatchSnapshot();
+});
+
 test('magic image', () => {
   expect(
     mount(