re-enable TPI test
While trying to refactor away some annoying atom count parameter in the PME code, I ran into TPI logic again, and realized it's not covered by tests.
While trying to throw together a test, I ran into "TPI does not work (yet) with the Verlet cut-off scheme" message.
Without at least a basic TPI sanity test, I doubt this feature will survive much longer...
Add a basic TPI sanity test
The floating point exceptions are for now toggled off
in the test body instead of mdrun itself.
Migrate TPI testing to Verlet scheme
For now, this also disables TPI testing, since TPI does not yet
support the Verlet scheme. If that gets fixed before this change gets
incorporated, the test should not be disabled.