gf field solve test fails with fewer than six processors

As @colinmroach notes, the gf field solve test fails when fewer than six processors are used. This is because the gf field solve only works when there are at least one processor per Fourier wavenumber; otherwise it exits with error.

Better behaviour would be for the test to be skipped when nproc < 6, rather than reporting failure.

Perhaps this should be on hold until we overhaul the testing infrastructure.