Make napari/tests a package

I considered making all the tests namespace packages, but it turns out pytest [chokes](pytest-dev/pytest#3396) [on](pytest-dev/pytest#1927) [namespace packages](pytest-dev/pytest#478).  So this is the best solution for now.

Test plan: made a distribution (python setup.py sdist) then installed it in a separate virtualenv.  then ran tests (`pytest --pyargs napari`).

Fixes napari#875