Add test for not uploading a non-updated file
Refactor file modification tests

* Change method names to use `modify` and `unmodified` instead of
`updated` and `non-updated`.
* Refactor test_modified_file_upload to not use `s3_file` variable.
* Update comments to reflect changes.
Move helper methods to bottom of file

Add comment separators for test methods and helper methods
Refactor file timestamp into helper method

Checking file timestamp happens a few times, so made it a method. Also
`time.sleep`ing for 2 seconds after second upload to let S3 catch up and
return the correct timestamp. Ugh.