sumpy.qbx is untested within sumpy

Follow-up issue from #1 (closed).

Changed title: {-sumpy.qbx-} is untested within sumpy → sumpy.qbx is untested within sumpy

mentioned in commit 95b2d355

mentioned in commit 6de49f50

6de49f50 added a test for Laplace but Helmholtz and others are still untested.