Fix delete user acceptance test

Adding some missing asserts showed that the "delete user" acceptance
test was silently failing, as the deletion was not being confirmed in
the dialog and thus the user was not being deleted.

The dialog button contains a single quote ("user0's"), so the XPath
expression had to be adjusted (it seems that it is not possible to
escape a single quote in a string enclosed in single quotes in XPath
1.0).

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