TestParseDuration relied on an elapsed time calculation which
would vary based on the system local time. Fix the test by not relying
on the system time location. Also make the test more deterministic
by injecting time in tests rather than using system time.
Fixes#4529.
The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.
The new code layout is documented in CONTRIBUTING.
