Fix least_squares result when xtol is reached

[winklemad]

Summary

• accept the Armijo-approved candidate before terminating on xtol
• add a regression test ensuring least_squares returns that accepted candidate

Problem

When an accepted step satisfies the relative step-size tolerance, least_squares sets DX_TOL and exits before assigning xnew and rnew. It therefore returns the previous, higher-objective iterate and records that stale iterate in the final trace.

Testing

• focused test_xtol_returns_accepted_candidate: passed
• complete python/mujoco/minimize_test.py module: 16 tests passed
• trailing-whitespace: passed
• end-of-file-fixer: passed
• git diff --check: passed

[kevinzakka]

Thanks for catching this!