don't fail on missing portal._original_mailhost attribute but just tear ...

...down the whole test stack

on my test setup, i'm missing the portal._original_MailHost attribute on test-teardown. the zope instance keeps hanging, and i have to `kill` it. the fix just tears down the test suite, when the _original_MailHost attribute cannot be found.

for my test setup see:
https://github.com/bluedynamics/bda.plone.shop/blob/master/src/bda/plone/shop/tests/__init__.py
https://github.com/bluedynamics/bda.plone.shop/blob/master/src/bda/plone/shop/tests/test_robot.py
https://github.com/bluedynamics/bda.plone.shop/blob/master/src/bda/plone/shop/tests/robot/test_shop_orderprocess.robot