GlIcon: Add `data-testid` attribute

Per this thread: gitlab!36240 (comment 378455487)

Occasionally we need to click on an icon in E2E tests. To avoid using CSS classes we should add a data-testid="{icon_name}-icon" attribute to GlIcon.

Update https://gitlab.com/gitlab-org/gitlab/-/blob/e044f3946a8773cdeeb4bc8b283ef12b906bfc52/qa/qa/ee/page/group/settings/general.rb#L69 once this is completed