Suppress notebook CI test failure when all tests are skipped

[ascillitoe]

In some circumstances, all the notebook CI tests are skipped. This causes a CI failure to be returned despite there being no problem with the notebook(s). For example, if a notebook excluded from some builds is changed, the test_changed_notebooks CI is run, but then all tests are skipped on some builds, as is the case here.

We should prevent the CI from being flagged as failing when pytest exits with code 5 (indicating all tests skipped). The pytest documentation recommends using the pytest-custom_exit_code plugin for this.