get rid of duplicated tests setup

[asmodehn]

We have two main sets of requirements :

• debs envs with:
  • dependency version based on rosdistro
  • tests dependency version based on rosdistro
• python envs with:
  • dependency version based on rosdistro
  • latest test dependency versions (from PyPI)

Since we extracted the tests from the main package for simplicity (especially for managing backward compatiblity and pure python testing), there is probably no need to have the tests dependencies version based on rosdistro. We should be fine using latest test dependencies.
This reduces the number of dependencies for the package, and forces us to do development work in a recent python environment (as per usual python workflow).

We will lose the possibility of running tests in old environments (unless we do pyros-dev/catkin_pip#104), and we would build ros package without tests... It might not be ideal, but saves quite some time and reduces complexity, especially because we can manage testing in multiple versions easily with tox and a set of requirements...

It means, we are relying on the linux distro to provide a consistent, up-to-date, python environment. That might be "good enough", and make our lives much simpler.
ROS packages then become a backport of python packages, which is the intent anyway.

So, with that in mind, it seems we can get rid of our deb envs requirement set...

[asmodehn]

This should be done in #14

[asmodehn]

PR has been merged, but this still remains...
It shall be done in parallel with ROS Third party release repo.