Expand unit testing

[seananderson]

Here's a start based on the last report in rough order of importance.

• [x] anisotropy with delta models
• [x] sdmTMB_simulate() with IID or AR1 or RW fields
• [x] add_barrier_mesh
• [x] mcmc residuals type
• [x] simulate() with delta models and model = 1 or model = 2
• [ ] some more cached parameter values, prediction values, and index values (add delta ones)
• [x] nlminb_loops
• [x] delta_truncated_nbinom1
• [x] tidy() with models that have breakpts
• [ ] sanity() with high gradients or NAs for SEs or a random field sigma < 0.001
• [x] wide format get_cog
• [ ] spread_sims basics
• [x] plot_pc_matern basics

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• [ ] Also, outside of the testthat folder (maybe within a folder within inst for now?): testing against VAST

Note, set:

Sys.setenv(NOT_CRAN = "true")

for covr to realize not on CRAN.