test: Verify NLL values and Minuit parameter uncertainties

[alexander-held]

Description

Resolves #1193.

This adds a check for the NLL value returned by minimization with various backends, which is compared against a reference.

It also adds a check for the parameter uncertainties returned by Minuit, which was motivated by #1183 (the addition of the HESSE call would have resulted in a slight change in the uncertainties returned here). A test for the parameter values could also be added here, but since that behavior is tested in test_minimize already I did not add it again here.

I noticed that three functions set parameters constant without using the keyword argument fixed_vals, which I expect is not on purpose. I added another commit in here that changes that.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Add a test for the NLL value returned by minimization
* Explicitly specify the fixed_vals kwarg for function calls passing in fixed parameter values

[matthewfeickert]

Thanks very much for the PR, @alexander-held! LGTM.