Failing tests are not signalled by GitHub

[ziotom78]

Old test runs like this show that failing tests are not correctly reported as such in GitHub Actions, which mistakenly marks them with «success»:

…
test/test_destriper.py::test_destriper FAILED                            [  1%]
test/test_detectors.py::test_detector_from_dict PASSED                   [  3%]
test/test_detectors.py::test_detector_from_toml PASSED                   [  5%]
test/test_detectors.py::test_detector_from_imo PASSED                    [  7%]
…

It seems that the problem is a lack of checks in tests/run_tests.sh (which is used by the test workflow).