This commit adds a test case for `test_find_best_app` where
Module object does not contain Flask application.
Also cleans the function little bit to provides more meaningful comment.
Function `prepare_exec_for_file` has a side effect where a path is added to
`sys.path` list.
This commit enhances an exisiting test case for `prepare_exec_for_file`
by testing the side effect of the function and adding necessary
comments.