Add main function to joint transmitted wrench test

[scpeters]

🦟 Bug fix

Fixes test coverage reduced in #495

Summary

I quickly approved and merged #495 that moved a test to a new file without realizing that it was missing the custom main function that is required for our common tests that allows passing the plugin library to the test from the CLI. There was reduced test coverage in #495 and I didn't realize why. This should fix it.

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 #496 (b2ed03d) into gz-physics6 (adde787) will increase coverage by 0.54%.
The diff coverage is n/a.

❗ Current head b2ed03d differs from pull request most recent head 66bc383. Consider uploading reports for the commit 66bc383 to get more accurate results

@@               Coverage Diff               @@
##           gz-physics6     #496      +/-   ##
===============================================
+ Coverage        75.02%   75.56%   +0.54%     
===============================================
  Files              142      142              
  Lines             7191     7191              
===============================================
+ Hits              5395     5434      +39     
+ Misses            1796     1757      -39     

see 3 files with indirect coverage changes

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