Add tests for ShaderType, GraphicsAPI, and DistortionPass

[iche033]

🦟 Bug fix

Summary

Added unit tests for ShaderType, GraphicsAPI, and DistortionPass, which currently have zero or very low test coverage.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #671 (67a24cc) into ign-rendering6 (a8e06ae) will increase coverage by 0.10%.
The diff coverage is n/a.

@@                Coverage Diff                 @@
##           ign-rendering6     #671      +/-   ##
==================================================
+ Coverage           54.92%   55.03%   +0.10%     
==================================================
  Files                 202      202              
  Lines               21102    21102              
==================================================
+ Hits                11591    11614      +23     
+ Misses               9511     9488      -23     

Impacted Files	Coverage Δ
...e/ignition/rendering/base/BaseGaussianNoisePass.hh	100.00% <0.00%> (+3.33%)	⬆️
src/ShaderType.cc	100.00% <0.00%> (+87.50%)	⬆️
src/GraphicsAPI.cc	100.00% <0.00%> (+100.00%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a8e06ae...67a24cc. Read the comment docs.

Addressed