Revamp tests; Delete outdated; Make common conftest.py; Improve segmentation tests; Allow segmentation tests to run on CI

Make code and tests flexible to the use of a pre-processed Montgomery dataset to speed-up testing where possible (closes #74)

Histograms comme be compared directly due to differences in how the
torch.resize transform is applied depending on the environment.
A statistical approach using pearson coefficients is used instead.
Testing of alexnet and densenet transforms have been temporarily
disabled as histograms or RGB images need to be handled differently.

[tests] Revamp tests for montgomery, shenzhen, indian, and tbx11k; Erase tests for outdated RS modules