PLATUI-1550: fix hmrc-header mobile dropdown behaviour

[oscarduignan]

This has been broken for any service using hmrc-frontend with a version of govuk-frontend after version 3 where they changed the code that initialises the behaviour of govuk-header (which hmrc-header depends on) to require data-module attributes to use a "govuk-" prefix

Fix was to add the prefix to the data-module attribute. I've done some searching and it seems like there are possibly only two services using navigation items with the hmrcHeader (at least via play-frontend-hmrc) so it might be that's why this bug managed to go undetected for a while, because few hmrc services were using the navigation items in the hmrcHeader.

I've added a visual regression test to catch any issues with this in the future.

Did a search for other uses of data-module attributes that could have been missed and nothing came up:

madetech@oscars-laptop hmrc-frontend % rg 'data-module="'
src/all.js

8:  const $AccountMenuSelector = '[data-module="hmrc-account-menu"]';
18:  const $UserResearchBanner = document.querySelector('[data-module="hmrc-user-research-banner"]');
23:  const $CharacterCounts = document.querySelectorAll('[data-module="hmrc-character-count"]');

src/components/header/template.njk
7:  <div class="govuk-header hmrc-header {{ params.classes if params.classes }} {{ 'hmrc-header--with-additional-navigation' if params.signOutHref or params.languageToggle }}" data-module="govuk-header"

src/components/account-menu/template.test.js
20:      const $nav = $('[data-module="hmrc-account-menu"]');
39:      const $nav = $('[data-module="hmrc-account-menu"]');
60:      const $nav = $('[data-module="hmrc-account-menu"]');

src/components/account-menu/template.njk
5:<nav id="secondary-nav" class="hmrc-account-menu" aria-label="Account" data-module="hmrc-account-menu">

src/components/character-count/template.njk
5:<div class="hmrc-character-count" data-module="hmrc-character-count" data-language="{{ language }}"

src/components/user-research-banner/template.njk
1:<div class="hmrc-user-research-banner" data-module="hmrc-user-research-banner">