session: execute browser teardown only once (#1390)

The browser teardown in `airgun.session.Session.__exit__` should be executed only once.

Because when `Session` is nested in multiple context managers(*),
the `__exit__` method is called for each one
and this causes that the second and further `webdriver.quit()` call will fail,
because the browser session was already stopped.

(*) for example in the robottelo `session` fixture in `robottelo/pytest_fixtures/core/ui.py`.

airgun/session.py

@@ -232,7 +232,7 @@ def __exit__(self, exc_type, exc_value, traceback):
  not risen not to shadow real session result.
  """
  if self.browser is None:
- # browser was never started, don't do anything
+ # browser hasn't been started or was already closed, don't do anything
  return
  LOGGER.info('Stopping UI session %r for user %r', self.name, self._user)
  passed = True if exc_type is None else False
@@ -242,7 +242,7 @@ def __exit__(self, exc_type, exc_value, traceback):
  except Exception as err: # - TODO: fix bare except
  LOGGER.exception(err)
  finally:
- self._factory.finalize(passed)
+ self.browser = self._factory.finalize(passed)
 
  def _open(self, entity):
  """Initializes requested entity. If this is first time session