Rely on `set -e` to make the scripts immediately exit on failure and on
`run-test-in-systemd` as called by the YAML testcase to signal failures to LAVA
appropriately.

This simplifies the code and also avoids inconsistencies between the testcase
names used inside the scripts and by `run-test-in-systemd`.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>