We've mostly phased out nose from our own usage, but it's still being

used for the extension test runner. The behavior is different enough

that we need a transition period to move people over.
In most cases, once extensions are configured in a way to make use of

pytest, they'll be able to just run pytest instead of rbext test,

but we also want latter to continue to exist.
This change adds a mode to the TestRunner to use pytest instead of nose.

That can be opted into for now, and in a future version we'll switch it

over and remove the nose mode.

Ran rbintegrations tests via pytest and our test runner.