Generate test result from cargo test

[petrikvladimir]

ref #3

Generate xml from captured stdout of cargo-test.

I did not find a way how to get captured stdout from colcon. Therefore, I implemented function check_call_return_stdout as an alternative to check_call. I would prefer a better solution for obtaining stdout - is there another way how to get captured stdout in colcon?

[luca-della-vedova]

Superseded by #39 which doesn't depend on nightly.
We should still revisit this approach once the json output is stabilized though