Add acceptance test for resharing a file with another user

To reshare a file there must be at least three enabled users in the
system; although it would be possible to run the steps to create a third
user in the scenarios that need it for convenience a third enabled user
besides "admin" and "user0" was added to the default setup.

In a similar way, a new step was added too to login as a given user
name, similar to the steps to log in as "user0" and as "admin".

Finally, another actor, "Jim", was introduced for those scenarios which
should be played by three standard actors (that is, without a special
configuration like "Rubeus").

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>