Test model and gradients evaluate at initial condition (Issue #3034)

[bbbales2]

Submission Checklist

• [x] Run unit tests: ./runTests.py src/test/unit
• [x] Run cpplint: make cpplint
• [x] Declare copyright holder and open-source license: see below

Summary

Fix for #3034

The 100 tries to initialize the model currently tries to evaluate log_prob without gradients and then the next block of code tries to benchmark the gradients. This was failing.

Instead in this pull we try the 100 tries evaluating gradients as well (since we'll need them eventually).

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company): Columbia University

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)