tests: Add configuration for pytest

By default, pytest considers test classes only if they're named
'Test*'; Nose also allows 'test_*'.
Configure pytest to allow the non-pep8 names as well.
ipatests.util.ClassChecker: Raise AttributeError in get_subcls

Pytest considers NotImplementedError on attribute access an error.
test_automount_plugin: Fix test ordering

Nose ran the `test_a_*` and `test_a2_*` tests in opposite order
than the source suggested. Fix this.
Use setup_class/teardown_class in Declarative tests

Pytest will consider each Declarative test individually, running
setup/teardown for each one.
Move the setup and teardown to the class level.