support matching tox environment for catkin_pip

[asmodehn]

We currently support running nose and pytest for testing.

We should add support to use tox directly, but testing only the environment passed as arguments.

The advantage is that we would be using the tox configuration, and we avoid needing a complex test configuration in CMakeLists.txt

[asmodehn]

One of the problems here, is to hook up with catkin expectations (xunit file created in a specific place for example).
It might be worth investigating if we can also be more accepting regarding arguments to nosetest and pytest... without disturbing too much the existing cmake API...

[asmodehn]

A good test candidate is https://github.com/box/flaky package : if we can run its tests (tox) we can release it on ROS...

[asmodehn]

broken templates there : https://github.com/asmodehn/flaky-rosrelease

[asmodehn]

One good argument for this is also that it would allow us to run tests, from catkin, but in a separate python environment (where we can get latest versions of tests library).

This works if we trust that a test passing on latest python env with latest library also means the distro python env will work fine -> we rely on linux distro to provide a consistent, up to date, python environment.

That usecase would allow us to develop packages in pure python (if we can get the dependencies from PyPI), and test in each distro, while NOT relying on old test libraries as package dependency, but only test dependencies, that we don't need to port as ros package, or maintain bwcompat for.

ROS packages using that feature will become simple backports of python packages, and main dev workflow should be python.

This eases maintenance load a significant bit...