RF: re-work bet tests - use external benchmark data

unit_tests/bet/4D.nii.gz

unit_tests/bet/T1.nii.gz

unit_tests/bet/T2.nii.gz

unit_tests/bet/bet_4d/feedsRun

unit_tests/bet/bet_A/feedsRun

unit_tests/bet/bet_A2/feedsRun

unit_tests/bet/bet_bias_and_neck_clean/feedsRun

unit_tests/bet/bet_custom_cog/feedsRun

unit_tests/bet/bet_custom_f/feedsRun

unit_tests/bet/bet_custom_radius/feedsRun

unit_tests/bet/bet_eye_clean/feedsRun

unit_tests/bet/bet_mask_only/feedsRun

unit_tests/bet/bet_negative_gradient/feedsRun

unit_tests/bet/bet_no_images/feedsRun

unit_tests/bet/bet_positive_gradient/feedsRun

unit_tests/bet/bet_robust/feedsRun

unit_tests/bet/bet_simple/feedsRun

unit_tests/bet/bet_surface_mesh/feedsRun

unit_tests/bet/bet_with_mask/feedsRun

unit_tests/bet/bet_with_mask_and_skull/feedsRun