test: raise library test coverage

[bconn98]

Fixes #352 + Adds a ton of tests

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 84.61%. Comparing base (c981ca4) to head (4a56b4e).

Files	Patch %	Lines
src/encode/json.rs	50.00%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #349 +/- ## =========================================== + Coverage 63.01% 84.61% +21.59% =========================================== Files 24 24 Lines 1560 1566 +6 =========================================== + Hits 983 1325 +342 + Misses 577 241 -336 ```

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[estk]

I think we should break this up into a few separate PR's, there are too many disparate changes here to review it without sitting down for at least a few hours... which is difficult to find these days.

Can you please open separate PR's with cohesive motif's? Some possible MR's

• Config tests
• Encoder tests
• Filter tests
• Integration tests
• Appender tests
• Compound appender tests

In each PR please:

1. Call out any changes to code outside of mod test & why its needed
2. for each test, if it is not obvious why it needs to exists, please add a brief description

[bconn98]

Broke into the following PRs:

• compound appenders
• config
• filter
• integration
• encode
• appender
• readme