Find ign program instead of ignition-tools

[scpeters]

🦟 Bug fix

Re-enable UNIT_ign_TEST

Summary

The find_package(ignition-tools) call was removed in #56, but this disabled UNIT_ign_TEST. Instead, search for the ign program directly with find_program and adjust test logic accordingly.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• 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

[codecov]

Codecov Report

Merging #57 (23cb74e) into ign-plugin1 (111ba95) will increase coverage by 4.10%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           ign-plugin1      #57      +/-   ##
===============================================
+ Coverage        95.71%   99.82%   +4.10%     
===============================================
  Files               15       15              
  Lines              584      584              
===============================================
+ Hits               559      583      +24     
+ Misses              25        1      -24     

Impacted Files	Coverage Δ
loader/src/ign.cc	96.00% <0.00%> (+96.00%)	⬆️

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[ahcorde]

Thank you @scpeters