To keep existing tests which emit RemovedInPytest4Warnings running, decided
to go with a command line option because:
* Is harder to integrate an ini option with tests which already use an ini file
* It also marks tests which need to be removed/updated in 4.1, when
RemovedInPytest4Warning and related functionality are removed.
Fix#3737
don't use autouse fixtures for now because it would cause a proliferation
and overhead for the execution of every test. Rather introduce a
node.addfinalizer(fin) to attach a finalizer to the respective node
and call it from node.setup() functions if the setup phase succeeded
(i.e. there is no setup function or it finished successfully)