Documented simulating an authentication by manual token creation.

Even though there are better ways of doing the same thing (like alternative authentication strategy for test environment), we're getting lots of questions on how to properly authenticate a user in a functional test by creating a token manually (see https://github.com/symfony/symfony/issues/5161).

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | >=2.0
| Fixed tickets | -