Part of the motivation for this test is to verify compatibility with
Scratch 2.0 / SB2 behavior, so we shouldn't change the project to an SB3
file without very good reason.
This new test verifies that clones and their associated threads are
cleaned up properly by the `delete this clone` block. The clones run two
stacks each: one which waits and then deletes the clone, and another
which includes a `forever` loop: this is to verify that the thread
running the `forever` loop is ended when the clone itself is deleted.
The project does this with two batches of clones to ensure there are no
problems with reusing array indices previously occupied by now-removed
threads or clones.