[testlib] add an default testcase.adminaccess (and use it for default session) This adminaccess is the new offical way to get connection, and request on a repo.

[testlib] introduce a RepoAccess class to easily create connection and request Each RepoAccess hold a session for a user and three helper function to help create Connection, ClientConnection of WebRequest related to this session. related to #2920299

[repoapi] make ClientConnection.__enter__ return self This allow the standard idiom:: with repoapi.connect(repo, login='babar', passwork='elephant') as cnx: cnx.execute(…)