Add CI job that fails PRs if tests are taking >1s

[DominicOram]

All unit tests should be fast running and so not take >1s each. If they are taking longer it is most likely due to an issue with poor mocking. To make sure we enforce this we should add a CI check on a PR that confirms that no tests individually took too long.

The durations can be found with pytest /tests --durations=0 but this isn't very machine readable so it may be worth a look around to see if we can get them out in a nicer way

Acceptance Criteria

• CI blocks PRs where an individual test is taking longer than 1s