This test was a bit tricky to convert because it subtly relied upon the
lazy evaluation of an Iterable.

The onDismissed from Dismissable happens during the animation phase of
the pipeline. Previously, the ScrollableList had already been built for
that frame but had not evaluated its Iterable yet. When we got to the
layout phase, ScrollableList evaluated its Iterable and saw the updated
version of dismissedItems.

A straightforward conversion to ListView calls toList() when building
the ListView, but that evaluates the iterable when buildTest() is
called, which is before the calls to pump and therefore before the
animation phase, meaning the Iterable sees the old value of
dismissedItems.

This patch fixes the test to use the normal setState pattern to signal
that state upon which the build depends has changed. Now, the
onDismissed callback happens during the animation phase and the
StatefulBuilder is marked as dirty via setState, which causes it to
rebuild the ListView and re-evaluate the Iterable, seeing the updated
version of dismissedItems.

This change also lets us replace the gratuious use of pumpWidget with
pump now that we use setState rather than pumpWidget to trigger a
rebuild.