Lack of a11y unit testing.

[harryhmc]

PR #6 was raised to solve an a11y issue in one of the components that was not caught by the built-in Storybook tests.

In order to prevent such problems in the future, it would be good to have unit tests covering the accessibility of each component in their respective test files.

[colinrotherham]

Linking this with #192 (comment)

Can we pick this up as part of #216?

All component HTML is tested by Axe (upstream) in NHS.UK frontend

Rather than repeat the tests here we should test if our HTML matches NHS.UK frontend