Add integration tests for transferring files of a user with a risky name

The files:transfer-ownership performs a sanitization of users with
"risky" display names (including characters like "\" or "/").

In order to allow (escaped) double quotes in the display name the
regular expression used in the "user XXX with displayname YYY exists"
step had to be adjusted.

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