Add automated tests

[eweitz]

This adds automated tests for gtf_to_gen_pos.py and dge_to_ideogram.py.

Tests do full runs through each module, verifying testing the input CLI and file outputs. Mock data is sampled from real data; large samples are tracked via Git LFS (Large File Storage), small samples are tracked directly in this repo.

Example test suite run:

cd tests/
pytest
============================================================================ test session starts ============================================================================
platform darwin -- Python 3.7.5, pytest-5.3.2, py-1.8.1, pluggy-0.13.1
rootdir: /Library/WebServer/Documents/ideogram-dge/tests
collected 2 items                                                                                                                                                           

test_ideogram_dge.py ..                                                                                                                                               [100%]

============================================================================= 2 passed in 0.19s =============================================================================