audio/wavpack: Re-enable testing

The tests can take a rather long time to complete, so I thought they
were hanging. Add a pre-test target to explain this, so people with old
hardware like myself know what to expect and for folks who have
WITH_TESTING enabled. Normally, regression tests don't take this long.
This is also a team maintained port, so pitfalls should be highlighted.

I also retract my statement of "test target has been failing for a while"
in the previous commit message. This was based on me testing the previous
version and seeing the same behavior of the wvtest process seemingly
hanging as I observed in the current version before review.

Reported by: diizzy, arrowd
Fixes: f8ba7a9305fc - audio/wavpack: Update to 5.9.0