This test tends to be brittle on Mac CI machines, which are
heavily loaded and bursty. Since the tests are only run as part
of the "extra tests" test set, this increase should not have
a significant impact on the total duration of CI runs.
A test runner already has a --durations option to print durations.
However, this isn't entirely satisfactory.
When there are many tests, this produces output spam which makes it hard
to find the test failure output. Nevertheless, it is helpful to be
informed of tests which are unusually slow.
Therefore, introduce a new option --min-duration that causes all
durations above a certain threshold to be printed. This allows slow
tests to be visible without mentioning every test.
