They were broken because the base class is testtools.TestCase.  We could fix
this with run_tests_with but I'd rather just not return Deferreds at all.

Supposedly the method cannot fail so we just drop the Deferred on the floor.
It feels a little off but maybe it's okay.

let hypothesis pick the multiplier and shrink it for us

existing_states() can be empty if you don't ask for at least one voucher

also `_VoucherInsert` is not unpackable

also there's some other value constraints required by the implementation, like
type and length of voucher string; satisfy those.

Before it just ended up with DeferredResource which proves nothing at all.
Now it requires a sufficiently non-error-y status code to be happy.

Also factor out the details since we would otherwise have four instances of
them now.

Maybe it always should be?  But configuring only PYTHONPATH is strictly easier
than configuring PATH and PYTHONPATH so I guess we'll let people do that.

By the time this code runs we know PYTHONPATH must be configured correctly
since we import stuff from allmydata elsewhere.