[colcon] Upstream test isolation to colcon test

[adityapande-1995]

It would be nice to upstream #277 to colcon, such that one can run tests in parallel with isolation.

[adityapande-1995]

I opened a draft repo here : https://github.com/adityapande-1995/colcon-test-isolated/tree/main, which adds a new test step action for python. Getting the "isolate" executable is tricky here, since colcon tries not to mix python and C++ code. Essentially we're creating a command line tool that is invoked using the test step action.

One option would be to write the test isolation stuff in pure python.

[adityapande-1995]

After a brief discussion with the ros team, looks like mixing C++ and python code would be tricky if mixed in the same colcon repo. I'll try to rewrite the isolation script in pure python, and use that in colcon using shell extensions, rather than using the python test step action, which would have to be replicated to bne used with cmake. The shell approach makes it more generic.

[adityapande-1995]

Made a pip installable pure python package to be used as a command line tool :
https://github.com/adityapande-1995/isolate-ros

[adityapande-1995]

Closing this as we have a new package released into rolling here : ros/rosdistro#39743