Add key publisher test

[jennuine]

Part of https://github.com/gazebosim/gz-sim/issues/1575

Summary

Added KeyPublisher_TEST to increase code coverage

Checklist

• [x] Signed all commits for DCO
• [x] Added tests
• [x] codecheck passed (See contributing)
• [x] All tests passed (See test coverage)
• [x] 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[bot]]

Codecov Report

Merging #477 (d96237e) into ign-gui3 (0948063) will increase coverage by 0.30%. The diff coverage is n/a.

@@             Coverage Diff              @@
##           ign-gui3     #477      +/-   ##
============================================
+ Coverage     73.51%   73.82%   +0.30%     
============================================
  Files            30       30              
  Lines          3270     3270              
============================================
+ Hits           2404     2414      +10     
+ Misses          866      856      -10     

Impacted Files	Coverage Δ
src/plugins/key_publisher/KeyPublisher.cc	100.00% <0.00%> (+43.47%)	:arrow_up:

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[jennuine]

@ahcorde what's the minor comment?