PTFM-13652: improved prefix bash variable

Summary: Added tests, and code, that implements the improved bash prefix variable. It takes patterns into account, and also the '.gz' suffix.

Test Plan: An additional test app, prefix_patterns, was added to the file_load subdirectory.

Reviewers: klai

Reviewed By: klai

Differential Revision: https://phabricator.dnanexus.com/D148