Remove gendered pronouns from test names

As recommended by, among other things, the [GOV.UK style guide](https://www.gov.uk/design-principles/style-guide#writing-gender-neutral-text).

Note: I left some instances of ‘his’ in `config/initializers/devise.rb`, as this was automatically generated from Devise, so I decided it was better to leave that, in case we need to merge it with a future version of Devise.