ENH: Verbose tests

[cookpa]

Print status of individual tests and trace system errors on exit.

[cookpa]

Example output

Running utils tests
test_abp_n4 (__main__.TestModule_bias_correction.test_abp_n4) ... ok
test_abp_n4_example (__main__.TestModule_bias_correction.test_abp_n4_example) ... ok
test_n3_bias_field_correction (__main__.TestModule_bias_correction.test_n3_bias_field_correction) ... ok
test_n3_bias_field_correction_example (__main__.TestModule_bias_correction.test_n3_bias_field_correction_example) ... ok
test_n4_bias_field_correction (__main__.TestModule_bias_correction.test_n4_bias_field_correction) ... ok
test_n4_bias_field_correction_example (__main__.TestModule_bias_correction.test_n4_bias_field_correction_example) ... ok
test_merge_channels (__main__.TestModule_channels.test_merge_channels) ... ok
test_crop_image_example (__main__.TestModule_crop_image.test_crop_image_example) ... ok
test_crop_indices_example (__main__.TestModule_crop_image.test_crop_indices_example) ... ok
test_decrop_image_example (__main__.TestModule_crop_image.test_decrop_image_example) ... ok
test_denoise_image_example (__main__.TestModule_denoise_image.test_denoise_image_example) ... ok

Much easier to see what's going wrong.

[coveralls]

coverage: 81.148%. remained the same
when pulling b75cf75 on verbose_tests
into 3b5383e on master.