Guard against re-entrant autobegin in Core, ORM

Guard against re-entrant autobegin in Core, ORM

Fixed bug in "future" version of :class:`.Engine` where emitting SQL during
the :meth:`.EngineEvents.do_begin` event hook would cause a re-entrant
condition due to autobegin, including the recipe documented for SQLite to
allow for savepoints and serializable isolation support.

Fixed issue in new :class:`_orm.Session` similar to that of the
:class:`_engine.Connection` where the new "autobegin" logic could be
tripped into a re-entrant state if SQL were executed within the
:meth:`.SessionEvents.after_transaction_create` event hook.

Also repair the new "testing_engine" pytest fixture to
set up for "future" engine appropriately, which wasn't working
leading to the test_execute.py tests not using the future
engine since recent f1e96cb0874927a475d0c11139.

Fixes: #5845
Change-Id: Ib2432d8c8bd753e24be60720ec47affb2df15a4a