Create automation for 'As a user I would like to be able to export a graph as a PNG #224'

[mwalker-scottlogic]

The feature implemented in As a user I would like to be able to export a graph as a PNG requires automation tests to check that the download functionality does not regress

A branch has been started here with a method to capture the downloaded image (see test 'Verify AQI graph can be downloaded as a PNG' in city_page.spec.ts).

Ideally, there will be a test for each chart. Still to be established - how to check it is correct, can playwright open the download and then do a snapshot test against a manually verified golden image? Or can we do a check that makes sure the download was successful / not corrupted