small modification to test_fft_operator.py

Status Job ID Name Coverage
  Test
passed #202679
test_min

00:03:03

passed #202680
test_mpi

00:05:55

passed #202681
test_mpi_fftw

00:06:28

passed #202682
test_mpi_fftw_hdf5

00:07:58

66.0%
 
  Release
passed #202683
release_image_master

00:05:54