Prevent mandatory jobs from being added to the list of rejected jobs (bugfix)

[pieqq]

Description

When user de-selects jobs in the UI, they may de-select mandatory jobs.
These jobs, even if de-selected, will still be added to the list of jobs
to run later on, and will be executed.

Therefore, they should not be part of the rejected jobs list.

This problem does not happen when using the match feature.

Resolved issues

While investigating #1557 , I found this other issue.

Documentation

Tests

Apart from the unit test being updated, I ran the following test:

Launch Checkbox using this launcher:

[test plan]
unit = com.canonical.certification::client-cert-iot-ubuntucore-22-automated
forced = yes

De-select every job and run the test.

Check the generated submission.json:

• mandatory jobs (for instance manifest) are executed (part of results), and they are not in the rejected-jobs list.
• all the other jobs that have been de-selected are in rejected-jobs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 48.03%. Comparing base (b965681) to head (2fcb7da).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1602   +/-   ##
=======================================
  Coverage   48.03%   48.03%           
=======================================
  Files         371      371           
  Lines       39852    39853    +1     
  Branches     6734     6735    +1     
=======================================
+ Hits        19142    19143    +1     
  Misses      19993    19993           
  Partials      717      717           

Flag	Coverage Δ
checkbox-ng	68.65% <100.00%> (+<0.01%)	⬆️

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