[repository] provide a .new_session entry point
The current .connect only returns a `sessionid` which must be given to
repo._get_session(...) to get the real session object.
Related to #4151635.
[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.