FRR prints uptime different based on how long it is. Prior to this
commit yanger expected uptime to be on the format HH:MM:SS which it is
as long as it's shorter than a day. If it's longer the format changes,
this commit handles this.
The commit also adds unit test data triggering the different formats
(<day, <week, >week).
Side note:
cli-pretty could be made to represent the data on "FRR format" again,
which is perhaps more human friendly.
Signed-off-by: Richard Alpe <richard@bit42.se>