Turn on floating point exception trapping for GCC in tests

Some simulations trap floating point exceptions to ensure that their
code is working correctly, and we want VTK-m to work correctly in their
code. To check this, we want to turn on floating point exception
trapping in our test code. This is very implementation-specific, so for
now we are just turning it on for GCC. This will at least alert a
problem on some of the dashboards.