test: modularize R tests with helper functions and fixtures

[Bai-Li-NOAA]

What is the feature?

• This PR addresses issue Make the R tests more modular #606 by making the R tests more modular and removing redundant code from the existing "estimation test of fims." The modifications reduce the integration tests from ~900 lines of code to ~300 lines.

How have you implemented the solution?

• Added test data to the tests/testthat/fixtures folder and used this test data across multiple tests (e.g., tests/testthat/test-integration-fims-estimation.R and tests/testthat/test-parallel-with-snowfall.R)
• Created two helper functions in tests/testthat/helper-integration-tests-setup.R to set up the FIMS model and validate FIMS results using simulated test data

Does the PR impact any other area of the project, maybe another repo?

• No. It only affects R tests. Developers can refer to the documentation in tests/testthat/helper-integration-tests-setup.R and the examples in tests/testthat/test-integration-fims-estimation.R to set up additional integration tests.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.78%. Comparing base (39d0743) to head (fbcd878).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #651      +/-   ##
==========================================
- Coverage   78.88%   78.78%   -0.11%     
==========================================
  Files          36       36              
  Lines        1975     1975              
  Branches      141      141              
==========================================
- Hits         1558     1556       -2     
- Misses        374      376       +2     
  Partials       43       43              

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[iantaylor-NOAA]

Looks good @Bai-Li-NOAA.
Nice to see so much redundant code getting reduced and documentation improved.