Utilize subtests in test_download_logs.py (#995)

Summary:
Pull Request resolved: #995

# This stack:
* Adding tests for download_logs/
# This diff:
* I know it looks scary, but the vast majority of it is just tabbing...
* Every test is split into multiple sub-tests as per marksliva's recommendation in [this comment](https://www.internalfb.com/diff/D36819724?dst_version_fbid=1118320549024875&transaction_fbid=532330348543619).

Reviewed By: marksliva

Differential Revision:
D36863858

LaMa Project: L1089718

fbshipit-source-id: d42a423f758102c58ef86b9bf0b023cac4bf2876