test: Test cam-service with multiple application

Test the cam-service with two cam-app-example application
running concurrently, implement the test in test_static_config.py.

To do that, some changes needs to be made to the CAMComponent
and the pytest fixture functions, so add a new fixture function
to pass a second instance of the cam-app-example and modify
the CAMComponent to allow a change in the log name and to return
the process exit code through the stop method.

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