Add simple (failing) unit test (run via npm run test)

[ahx]

Added a simple unit test to ease further development. You can run the test via npm run test.

The tests are failing in master, but merging it now should help fixing the problem. Here's the current output:

AssertionError: '>' == 'Button'

AssertionError: 'Your standard form button.\n @state :hover - Highlights when hovering.\n @state :disabled - Dims the button when disabled.\n @state .primary - Indicates button is the primary action.\n @state .smaller - A smaller button\n @markup\n   <button>This is a button</button>' == 'Your standard form button.'

[darcyclarke]

👍 Love it, thanks @ahx