[devtools] remove the remaining bw compat for old-style tests
It is just:
* dangerously untested right now
* on the path of further bw compat removal (cnxset handling)
webtest drops an apparently pointless .login call
Related to #3933480.
[transactions] cleanup after the dbapi removal
* end req vs cnx dichotomy
* Transaction takes a cnx at __init__ time
* a couple of super calls are fixed
Related to #3933480.