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.
Merge pull request #170 from alphagov/remove-gendered-pronouns

Remove gendered pronouns from test names