Make it easier to spot when slow tests are added

[coretl]

We should make sure that all new tests are relatively fast, with a few smoke tests that are allowed to take a bit longer. I suggest a default per-test timeout of 0.5s, with an override for the few tests that take longer than this.

This would involve:

• Adding pytest-timeout as a dev dependency in pyproject.toml
• Setting in timeout = 0.5 in [tool.pytest.ini_options] in pyproject.toml
• Adding @pytest.mark.timeout(<a_longer_timeout>) for those tests that now start to fail

As per https://pypi.org/project/pytest-timeout/

In a later PR we will make the slow tests faster