Tests for updating the display configuration while the server is running

[mattkae]

What's new?

• Added a single test for updating the display configuration while a display server is running
• This does not work with ubuntu frame or mir-kiosk as they don't respect the environment variable
• We can add more tests if you think this is a viable route 👍

[codecov]

Codecov Report

Merging #59 (18d4c2c) into main (73e6880) will increase coverage by 0.88%.
Report is 1 commits behind head on main.
The diff coverage is 86.95%.

@@            Coverage Diff             @@
##             main      #59      +/-   ##
==========================================
+ Coverage   63.41%   64.29%   +0.88%     
==========================================
  Files          10       11       +1     
  Lines         697      717      +20     
  Branches       85       87       +2     
==========================================
+ Hits          442      461      +19     
  Misses        233      233              
- Partials       22       23       +1     

Files	Coverage Δ
mir-ci/mir_ci/display_server.py	78.12% <100.00%> (+0.70%)	⬆️
mir-ci/mir_ci/output_watcher.py	83.33% <83.33%> (ø)

... and 1 file with indirect coverage changes

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[Saviq]

Some tweaks inline

[AlanGriffiths]

We need more tests like this

[Saviq]

Not sure why you needed the decorator on each test rather than the class, but OK :)