Add unit tests for various blocks

[johngodley]

Some of the blocks here are used in fundamental page layouts. For example, the layout grid. As such it's important that any changes we make don't break or affect these layouts in any way.

We need to introduce some tests so that we can:

• verify the HTML output of the blocks
• verify the browser output of the blocks

In the case of the layout grid there are several options that behave differently depending on whether viewed on a desktop, tablet, or mobile, making it difficult to check manually.