Add `BEST EFFORT` controller switch tests for Controller Manager

[destogl]

We could probably use parameterized tests for this to save some code.

Originally posted by @destogl in #461 (comment)

Description:

• File: controller_manager/test/test_controller_manager.cpp
• each test where controller switching is done has to be done also with BEST_EFFORT - this means that the test will have to be duplicated (we can optimize/reduce code in the second step)
• one example line with STRICT controller switch is here
• BEST_EFFORT means the following: if we have a situation where multiple controllers "are switched" and can not be switched the all other that can will be still switched
• this feature is not documented very well - please also add documentation into controller_manager/doc/userdoc.rst

[Xi-HHHM]

Please assign to me.

[christophfroehlich]

@Xi-HHHM and I talked yesterday about further steps and that would be:

• split current test into "lifecycle test" and "activating multiple controllers when one is unknown"
• add test for "activating multiple controllers when one has a resource collision"

Originally posted by @destogl in #582 (comment)

[ashwinsnambiar]

@christophfroehlich Can I take up this issue? I am not very familiar with building tests, but I would like to take this opportunity to learn and contribute to the community.

[christophfroehlich]

Very welcome! Writing tests is mostly understanding the intention of the code itself, and copy-pasting other tests ;)
Don't hesitate to ask if something is unclear (I have to read up on this topic as well, I haven't looked into detail yet). But maybe @saikishor can also help fast here.

[saikishor]

Hello!

https://github.com/ros-controls/ros2_control/blob/master/controller_manager%2Ftest%2Ftest_controller_manager_srvs.cpp

In the above file, there are some tests already present. Better take a look at some coverage builds and understand what else needs to be implemented. I already see some tests with BEST_EFFORT.