New plainbox unittests (>0%)

[Hook25]

Description

This PR aims to make every module in plainbox at least imported by the unittests. This allows tox, pytest and any other static analysis tool we may want to start using to analyze them.

I have used this occasion to try to produce some reasonable unittests, so that we get also some value more than just a plain include.

Resolved issues

This is part of: https://warthogs.atlassian.net/browse/CHECKBOX-740

Documentation

N/A

Tests

To run the tests run the following

$ cd checkbox/checkbox-ng/plainbox
$ pytest . -vvv

[codecov]

Codecov Report

Merging #675 (f69c001) into main (a4416bb) will increase coverage by 27.21%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             main     #675       +/-   ##
===========================================
+ Coverage   31.46%   58.67%   +27.21%     
===========================================
  Files         302      103      -199     
  Lines       35156    14430    -20726     
  Branches     6141     2374     -3767     
===========================================
- Hits        11061     8467     -2594     
+ Misses      23572     5632    -17940     
+ Partials      523      331      -192     

see 204 files with indirect coverage changes

[kissiel]

Awesome stuff! +1
Is this really everything that was needed to import all modules while testing?

[Hook25]

this imports everything in plainbox but plainbox/impl/box.py because it is more trouble than it is worth fixing it I'm worried.