REGTESTS: http-rules: test the HTTP authentication action, fetches and ACL

REGTESTS: http-rules: test the HTTP authentication action, fetches and ACL

Neither the "auth" action nor the http_auth_user, http_auth_pass,
http_auth_type and http_auth_group fetches were covered, and the
userlist section was only exercised through the stats.

The test declares a userlist with two users, one of them in a group,
and checks that the credentials are readable before being validated,
that http_auth() only matches with the right password, that
http_auth_group() only matches for the member of the group, and that
the "auth" action emits a 401 carrying the requested realm until valid
credentials are presented.