chore: Split big chunks of code in `sylvia_derive::multitest`

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.54839% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 84.71%. Comparing base (8c63021) to head (d560dab).

Files	Patch %	Lines
sylvia-derive/src/multitest.rs	93.54%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #360 +/- ## ========================================== + Coverage 84.60% 84.71% +0.11% ========================================== Files 37 37 Lines 2578 2598 +20 ========================================== + Hits 2181 2201 +20 Misses 397 397 ```

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