Break YamlReadingTest into smaller parts

[charleskorn]

This file has become huge (2000+ lines 😱), making it difficult to navigate and slow to work with in IntelliJ.

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[russellbanks]

Kotest doesn't support multiplatform nested tests due to limitations in Javascript, so the tests in commonTest won't actually run on all platforms unless they are refactored to not be nested.

kotest/kotest#3141 (comment):

The Javascript test runners don't support promises at the outer layer.

Therefore we cannot use coroutines anywhere but the final scope. Since kotest supports coroutines at every layer, the only way to make it work in JS land is to support flat tests only.