pull: Add support for resuming downloads via range requests

Use a consistent temporary filename to download uri's.
Check for downloaded files before fetching from uri.
Download to hash.part file, then copy/move to hash.done when complete.
Add argument support to setup_fake_remote_repo1 function.
Add test for pull resume.
To implement this, pass --force-range-requests into the trivial-httpd,
which will only serve half of the objects to clients at a time.

https://bugzilla.gnome.org/show_bug.cgi?id=706344