Update tests to work with core Nengo

[hunse]

Update tests to work with new nengo development version.

TODO:

• [x] Replace the old --neurons argument to specify the neuron types for Nengo tests with something.
• [x] Skipping unsupported Nengo tests does not seem to be working.
• [x] Backwards compatibility (if we want it/if it's possible)

[hunse]

We could also support newer pytest. One problem right now is that we have a number of tests that get skipped if the --target option has a particular value. We used to mark these tests with something like @pytest.mark.skipif(pytest.config.getoption("--target") == "loihi"), but there's no pytest.config anymore. I guess we could move this check inside the test, and use request to get the config value.

[drasmuss]

The problem I've had upgrading pytest is that if you still want to support the nengo 2.8.0 tests then you're stuck with pytest<4.1. We could try to install different pytest versions during CI depending on which nengo version we're testing against, but that makes things more complicated (and we probably have to end up complicating our test suite supporting the two different versions). So I'd probably not bother until we want to increase the minimum nengo core version to something that is also compatible with newer pytest.

[drasmuss]

Also re:

Replace the old --neurons argument to specify the neuron types for Nengo tests with something.

There's a nengo_neurons pytest config option now for setting that (but we'll still have to use the old --neurons method when testing against 2.8.0).

[tbekolay]

I've updated this PR to work with the TravisCI jobs that target Nengo 2.8 and Nengo master. The full list of changes can be seen in the commit message of df4d54f9868eaa938497f28473ab6621fae92208.

This is now ready for review.

[tbekolay]

Also, I don't believe that any of these changes need changelog entries as these are all changes to infrastructure and testing.