Fix issue detected by ros2 jenkins ci

[fspindle]

• see https://build.ros2.org/job/Idev__visp__ubuntu_jammy_amd64/88/
• fix unused functions in testColorConversion.cpp when isolated build that doesn't use dataset

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4851925) 53.60% compared to head (2d6eb6c) 53.60%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1314   +/-   ##
=======================================
  Coverage   53.60%   53.60%           
=======================================
  Files         399      399           
  Lines       50509    50509           
=======================================
  Hits        27076    27076           
  Misses      23433    23433           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.