Fix tests failing with new OpenMP on Intel compiler

- NearestNeighborFieldInterpolator::get needs to break out of all loops, just not the current one. While the test seemed to succeed anyway, it crashed with corrupted memory during exit when compiled with Intel 17 in Release mode.
- FftTest should not use thread-unsafe RNG in parallel section. Otherwise the Intel 16 compiler crashes.
- Revert to -openmp on Intel < 16.0.3, which crashes while processing some of our OpenMP loops (e.g. BoundaryHandling::treatDirection).