test: add SelfReferentialStruct test case

[puckey]

Adds a struct with a callback that references the same struct type.

Currently fails with "Maximum call stack size exceeded" due to infinite recursion in nitrogen's type walker.

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[mrousavy]

We need to add https://nitro.margelo.com/docs/types/custom-structs#cyclic-references-are-not-supported

[puckey]

For this case I expected it to be possible, or is it something you don't want to support:

export interface SelfReferentialStruct {
  transform?: (config: SelfReferentialStruct) => Promise<SelfReferentialStruct>
}

[puckey]

Closing in favor of #1074 which includes this test case along with a fix