[testlib] move repo and related attribute back on Instance instead of Class The repo and cnx was hold by the TestCase class to work as a cache and avoid recreation of the repo from scratch of each test. However since bad26a22fe29 the caching is done by the DatabaseHandler object and it is not necessary have a second layer of cache on the TestCase itself. We move the repo and cnx attribute back on the TestCase instance itself and make several class methods instance methods again. This will helps to change dbapi access in test from dbapi to repoapi. related to #2503918

[dbapi] makes anonymous_connection a computed property The current implementation is a boolean flag set manually by client code after connection creation. This led to different way to decide a anonymous_connection should be True (eg. different in the test than in the actual application code). It should not be client responsibility to set this flag. ``cnx.anonymous_connection`` is now a purely computed property. Connection with user in the "guests" group are anonymous, the other ain't. ``Session.anonymous_session`` is computed from ``cnx.anonymous_connection`` and get the updated behavior transparently. Closes #2953943