fix: print proper estimated runtime even when some jobs don't provide it

[kissiel]

Description

Drive-by fix for the estimated duration computation that Pierre complained about yesterday.
With this patch, if a job exists that doesn't specify the estimated_duration, Checkbox will still compute the whole estimated runtime but it will add the "at least" clause.

I tested it by creating a provider with two jobs, one with a estimated_duration defined, and one that did not have it.
I've run sessions with varying selections to see if it works properly.

[pieqq]

LGTM

I've tested it by running com.canonical.certification::client-cert-iot-ubuntucore-22, which has jobs with no estimated_duration, and in this case I get an output like:

========[ Running job 5 / 97. Estimated time left (at least): 1:51:00 ]=========

If I run checkbox with a test plan where all the jobs have an estimated_duration, I get something like this:

==============[ Running job 1 / 88. Estimated time left: 0:56:33 ]==============