This thorn doesn't have a "test suite" in the usual Cactus sense of
this phrase.  However, the CactusTest arrangement contains (among others)
the thorns TestInterp and TestInterp2:
* TestInterp used to test CCTK_InterpGV() and CCTK_InterpLocal()
  interpolators, although it seems to be broken at the moment
* TestInterp2 tests CCTK_InterpGV(), CCTK_InterpLocal(), and
  CCTK_InterpLocalUniform() interpolators, but at the moment it's
  not integrated into the Cactus test architecture, i.e. you have
  to run it explicitly and look at the output to see how well things
  worked.