[devtools/testlib] Use actual 'admin' user configuration values
Previously ``db-user`` and ``db-password`` configuration values were used to
fill ``admlogin`` and `admpassword`` attributes of CubicWebTC. The correct
data for these should read from `[admin]` section.
Closes #4065070.
addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile