In the new model of gathering tests, the pytester plugin which
ensures that fixtures are started and stopped but not counted
as tests was not being run because test-name matching was no
longer working correctly.
These changes ensure that the plugin is run. Despite the small
change here, this took absolute ages to get right.
And it still doesn't fix the problem present in the previous
commit: The need for the test_pytest function to be present
in the namespace of the test module. So a few more iterations
remain.
17fbae067d