Can't run test_views_nodes.py in isolation

[Jeroen T. Vermeulen]

Running src/maasserver/tests/test_views_nodes.py by itself fails before any tests can execute:

    AlreadyRegistered: The model BootImage is already registered

The trigger for this seems to be this import statement:

from maasserver import messages

That import is needed only for TestGetLongpollContext, which seems like a bit of a subject onto itself. So we could shorten the test cycle for the node-related views considerably by moving that test out into its own file!