[BUGFIX] Assure fresh session for functional tests
The changes done to resolve #35965 came with a functional
test that worked fine when run in isolation but failed when
run with other tests that would start a session.
This patch moves the $session->destroy() call in the
base test case from the tearDown() to the setUp()
method and fixes the behavior of destroy() in the
TransientSession which did not reset the "started"