test: Improve CAMComponent stop method and fixture

Improve the CAMComponent stop() method by checking if the
process is running and using wait() after sending the
termination signal, because it is not guaranteed that the
process will stop after that.
Use kill() as last resort if the application is still
running.

While there, improve the conftest.py fixture function,
there is no need to specify an autouse function to stop
the server, while we can use the yield in its fixture
function as explained in the docs.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>