Add SS3 integration tests for testing convert_output()

[Bai-Li-NOAA]

This PR addresses issue #29

• tests/testthat/fixtures/prepare_ss3_output_files.R downloads and runs SS3 test models from the ss3-test-models repository. After running the models, all files are deleted, except for the Report.sso files, which are saved for testing ASAR.
• tests/testthat/test-convert_output.R tests the convert_output() function using all SS3 test models saved in tests/testthat/fixtures/.
• The tests verify that no errors occur when running convert_output(), ensure the result contains 31 columns, and confirm the file_save flag works as expected. Feel free to add any additional tests if needed.

All tests have passed.

[Bai-Li-NOAA]

It won't affect the tests, but thanks for flagging it. There was a race condition where SS3 hadn’t finished saving reports before the code deleted irrelevant files. Since SS3 runs externally, the deletion sometimes happened too soon. I fixed it by moving the file deletion out of the parallel process.