Chamel/unit testing and coverage

[cmhamel]

This PR does a few things but the main goal is to separate the unit testing code from the source code for easier code coverage to track down untested and even dead code.

1. Move test files from optimism/ to ../test organized by submodule.
2. Change keyword tol to rtol in cg method calls in tests for the inverse module due deprecations.
3. Hook up a code coverage badge in README and ci yaml files.

[cmhamel]

Oof 78 files changed!

Note that all files changes except for in the inverse module testing, the changes are import statements due to moving the testing fixtures around.

[codecov]

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