cubicweb #4065070 CubicWebTC uses db-user configuration instead of admin/login,password [validation pending]

In init_config, the admin user login and password are stored as admlogin and admpassword attributes. Corresponding data is read from db-user and db-password configuration values which have nothing to do with the admin user. This should instead be read from the admin section. This particularly matters for Postgres tests where the db- options are set.

done in3.20.0
load left0.000
closed by#58c7a075c793 [devtools/testlib] Use actual 'admin' user configuration values
patch[devtools/testlib] Use actual 'admin' user configuration values [applied]