Remove usages of `io::rdump` from unrelated tests, util functions

[WardBrian]

Submission Checklist

• [x] Run unit tests: ./runTests.py src/test/unit
• [x] Run cpplint: make cpplint
• [x] Declare copyright holder and open-source license: see below

Summary

We've been recommending against using the R dump format for a while (see https://github.com/stan-dev/cmdstan/pull/1177), but we were still using it for a lot of tests (sometimes completely unnecessarily), and for the two helper functions create_unit_e_diag_inv_metric and create_unit_e_dense_inv_metric, which were both implemented by creating a string and then parsing it again.

Intended Effect

Clean up unnecessary usages of io::dump in tests, replace others with json data, and refactor utilities that don't need to do parsing to not do it.

How to Verify

Side Effects

Documentation

Copyright and Licensing

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)