Feat update testing fixtures

[MOshima-PIFSC]

Changes included in this PR:

• updated unit tests so they call the simple_small SS3 model from r4ss (system.file("extdata", "simple_small", package = "r4ss")
• added a copy of the simple_small model input files (starter.ss, forecast.ss, control.ss, and data.ss) to the inst/extdata folder
• in github action, retrospectives will be run by downloading the ss executable file, running in the test script and then deleting all files after tests are complete

These changes will move us toward being able to remove the large .rda files (simple.rda and retroSimple.rda) in the future. The next step will be during the documentation milestone to update the vignettes to use these structures.

This new workflow should replace the current call-r-cmd-check.yml workflow.

[efletcherPIFSC]

@MOshima-PIFSC The r package checker found errors in the test code, but it was due to checking if stock synthesis was installed in my system,

r4ss has a test_that method that skips the test if the stock synthesis executable is not on the system: https://github.com/r4ss/r4ss/blob/9d592166e3415f61875a9e19bc0741c0a0c5a5b0/tests/testthat/test-runs.R#L20)

[efletcherPIFSC]

R-package check with bugfixes resolves #89 issue.