Add tests for Optim.BFGS() with user-supplied grad and bounds

[sjdaines]

Fixes https://github.com/SciML/Optimization.jl/issues/760 https://github.com/SciML/Optimization.jl/issues/755

• add tests for Optim.BFGS() with user-supplied grad and bounds
• fix wording for error message if bounds specified without either grad function or AD

Checklist

• [ x] Appropriate tests were added
• [ x] Any code changes were done in a way that does not break public API
• [ x] All documentation related to code changes were updated
• [ ] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [ x] Any new documentation only uses public API

Additional context

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 0.06%. Comparing base (e2cad3d) to head (19da423). Report is 83 commits behind head on master.

Files	Patch %	Lines
lib/OptimizationOptimJL/src/OptimizationOptimJL.jl	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #761 +/- ## ========================================== - Coverage 63.85% 0.06% -63.79% ========================================== Files 22 22 Lines 1527 1605 +78 ========================================== - Hits 975 1 -974 - Misses 552 1604 +1052 ```

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