Add tests to increase test coverage

[Bi0T1N]

🦟 Bug fix

Summary

Increases the test coverage by adding tests for uncovered code parts. First I checked the older branch for Color.cc but it shouldn't harm to add it anyway (test the copy constructor). The other test is based on gz-math7 forMaterial.cc.

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 #533 (1069f3b) into gz-math7 (836a5c5) will increase coverage by 0.04%.
The diff coverage is n/a.

❗ Current head 1069f3b differs from pull request most recent head 2441be8. Consider uploading reports for the commit 2441be8 to get more accurate results

@@             Coverage Diff              @@
##           gz-math7     #533      +/-   ##
============================================
+ Coverage     99.70%   99.74%   +0.04%     
============================================
  Files            77       77              
  Lines          7031     7032       +1     
============================================
+ Hits           7010     7014       +4     
+ Misses           21       18       -3     

see 2 files with indirect coverage changes