Add pytest image marker

[bjlittle]

🚀 Pull Request

Description

This pull-request adds the image marker for pytest to identify plotter rendering image tests.

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[codecov]

Codecov Report

Merging #485 (eb53059) into main (e652eb5) will increase coverage by 0.00%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #485   +/-   ##
=======================================
  Coverage   90.74%   90.74%           
=======================================
  Files          70       70           
  Lines        3684     3686    +2     
=======================================
+ Hits         3343     3345    +2     
  Misses        341      341           

Files	Coverage Δ
tests/plotting/test_examples.py	100.00% <100.00%> (ø)