EpiNow2: Expand unit testing coverage and explore edge cases

[benjaminortizulloa]

please copy in previous

EpiNow2 has partial unit tests but these do not cover all edge cases and are often at a fairly high level. Expanding unit testing to cover more of the package code and to be lower level would greatly improve the rigour of the code and prevent future development causing issues.
Tasks:

• Explore current unit tests
• Highlight areas where testing is weak or not present
• Add low level unit tests to plug these gaps
• If possible improve the testing of the stan models behaviour when fit to data but this may be complex.

[impact: please copy in previous]
[originally proposed by @seabbs]
[suggested repo: https://github.com/epiforecasts/EpiNow2 | @seabbs]
[additional notes: ]