`pyFAI.test.test_mask.TestMask` waste resources

[kif]

Measured about 7 seconds, performs ~20 azimuthal integation on 1Mpix images over 1000 bins.

Idea: replace the image with a 100kpix image and 100 bins. Should make the test below 1second (to save energy & time).

This requires to regenerate a fake diffraction image with a circular negative ring (value =-10). Could be done during the test or beforehand using fake_alibration_image