Add smart brim to leg test object and update test instructions

Issue #264 (closed) suggests adding the smart brim to the test print and also possibly test other aspects of printed parts.

• Add smart brim to leg_test.stl
• Update test your printer to include the smart brim
• New images of the printed test leg, one oriented as before, one from below showing the brim
  * [ ] Add mention of the nut trap test object This does not make sense without instructions for testing a nut in the nut trap. Testing trapping a nut probably requires a different shape body to make it easier to grip and would require new renders.

closes: #264 (closed)