Report invalid units generated during test session (bugfix)

[Hook25]

Description

Currently if a template generates a unit that is invalid, a message is logged (error for some, debug for some others) and the unit is skipped. This message is visible using Checkbox local but it burried in the Agent log in remote. This is of course sub-optimal as we have no recollection of anything happening after the fact but this can lead to tests being... not executed, without an explanation or even a way for the user to notice.

This PR introduces a new unit, the InvalidJob unit, that allows us to report the invalidly generated units as failures, so that they are present all reports and on C3. This change is made along the new feature section, that allows us to toggle features like this on or off (to allow people to transition into big changes instead of having to update forcibly). The changes are both in this PR because the feature change is about 4 lines, and it makes sense to create and use it (else, I don't see how I could test it)

Resolved issues

Fixes: CHECKBOX-1677
Fixes: CHECKBOX-1676

Documentation

Pending implementation approval, I think this should be mentioned in both the extended tutorial and the normal tutorial

Tests

This implements a new metabox scenario and unit tests

[codecov]

Codecov Report

Attention: Patch coverage is 97.61905% with 3 lines in your changes missing coverage. Please review.

Project coverage is 48.90%. Comparing base (d0c208e) to head (0c04dae).
Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
checkbox-ng/plainbox/impl/unit/job.py	96.22%	2 Missing ⚠️
checkbox-ng/plainbox/impl/execution.py	90.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1560      +/-   ##
==========================================
+ Coverage   48.80%   48.90%   +0.09%     
==========================================
  Files         369      369              
  Lines       40192    40258      +66     
  Branches     6783     6799      +16     
==========================================
+ Hits        19616    19687      +71     
+ Misses      19857    19852       -5     
  Partials      719      719              

Flag	Coverage Δ
checkbox-ng	69.38% <97.61%> (+0.17%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[pieqq]

Great stuff!

A few typos and other small comments, but otherwise good to land I think!

[pieqq]

Much clearer, thanks!