Expand unit testing

[seananderson]

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

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