test,doc: Don't use exit fault action in the Pytest

The exit fault action makes the service exit in case of errors,
unfortunately there are a limited amount of logs that are printed
before exit. In order to ease the debug in case of errors, remove
the exit action from the Pytest and instead check for error strings
inside the service log.
Because of this change, the test_cam_app_fault_injection test is
split in two, in order to have two separate log.

Furthermore, in order to test the exit fault action which would
remain untested given this change, a new test is introduced.

Update the documentation with the new number of tests.

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