Hey,
I want to to the following:
For some research I want to change some code in the AP_Motors Library.
I am then running tests with the autotest framework of ardupilot.
Is running the build.Copter step before test execution enough to enable the code or do I have to do something else to get my code running on the board (it’s code that is not very easy to observe so I don’t really know if my changes took effect yet).
Sorry for the nooby question 
'That should be enough, it will recompile the code.
That will work. Though by board I assume you mean SITL
BTW you can use --no-clean if you are making small changes so you dont recompile everything each time. Here is my common command,
I run the tests in a higher level folder.
./../ardupilot/Tools/autotest/autotest.py build.Copter test.Copter.Mount --no-clean
If you’ve done major changes prepending removal of the binaries can be useful.
rm ../ardupilot/build/sitl/bin/* & ./../ardupilot/Tools/autotest/autotest.py build.Helicopter test.Helicopter.AirspeedDrivers
You can also debug the Autotests in VSCode (both C++ and Python) Though C++ is tricky at the same time.
https://ardupilot.org/dev/docs/debugging-with-gdb-using-vscode.html