fix(testing): use cookie for testing authentication

[SychO9]

Fixes #3834

Changes proposed in this pull request:

• The authenticatedAs testing option makes use of an authorization header to authenticate the user, this only works on the api frontend, for forum or admin frontends we've always had to make a normal identification/password request to the login endpoint and use the generated cookie to run our tests. This isn't ideal.
• This PR changed the autheticatedAs option to rely on a remember cookie by default. Which makes testing easier.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Tests have been added, or are not appropriate here.