bullet-featherstone: Reset joint motor constraint's velocity target after each step

[iche033]

🦟 Bug fix

Fixes gazebosim/gz-sim#2654

Summary

Resets the joint velocity cmd after each step to be consistent with dart behavior.

Note: I also tried removing the joint motor constraint after each step but that didn't work (test fails) so opted to reset the joint velocity target to 0.

We have a test for this in joint_features but only for dart. This PR enables the test for bullet-featherstone as well. There is also a test in gz-sim in a branch (see instructions in gazebosim/gz-sim#2654). The test there should now pass with these changes.

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)
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