the test waveinterp_2p sets up initial data using IDWaveMoL (fom CactusExamples) using kx=ky=kz = 0. and slopet = 0.1. Ie the initial data is constant in space and has frequency 0.1. It then evolves this using WaveMoL and interpolates in space and time with CarpetInterp.
Since the function is constant in space (exactly so, see eg the output in InterpToArray::array1d_vars) the interpolation in space just tests roundoff errors. The second time derivative (array1d_vars) should be zero and all we measure in the test is truncation errors on the level of 1e-9. This makes the test very sensitive to -Ofast, -O0, gcc vs. intel etc. and not a very good test.
I would rather either use a Gausian wave for initial data so that there is some actual data in the result and not just numerical noise, or leave out the 2nd time derivative from the test data.
The test fails due to significant errors in a virtual machine using ubuntu.cfg (which usies -ffast-math and -O2).