This means that e.g. for `TEST_CASE` with two sibling `SECTION`s
the event will fire twice, because the `TEST_CASE` will be entered
twice.
Closes#2107 (the event mentioned there already exists, but this
is its counterpart that we also want to provide to users)
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.