output more sensible error

tests/misc/run.sh

 make
-mpirun -np 4 -x OMP_NUM_THREADS=1 test_fftw
+mpirun -np 2 -x OMP_NUM_THREADS=1 test_fftw

tests/misc/test_fftw.c

@@ -305,8 +305,8 @@ int main(
     //L2norm1 = sqrt(L2norm1 / (nx*ny*nz));
     //L2norm2 = sqrt(L2norm2 / (nx*ny*nz));
 
-    printf("L2normk = %g, L2norm1 = %g, normed_diff = %g\n",
-            L2normk, L2norm1, fabs(L2normk - L2norm1)/npoints);
+    printf("L2normk = %g, L2norm1 = %g, relative error = %g\n",
+            L2normk, L2norm1, fabs(L2normk - L2norm1) / (L2normk));
 
     // deallocate
     fftwf_destroy_plan(r2c_plan);