chore: reorganize test folder structure

[maximilianfalco]

	Fix RM-XYZ

🧰 Changes

Final step (🤞) in the mdxish cleanup. Just reorganizing test folder structure so its easier to navigate and read.

Before

__tests__/lib/
├── __snapshots__/
│   ├── mdx.test.ts.snap
│   ├── owlmoji.test.ts.snap
│   └── stripComments.test.ts.snap
├── exports/
├── mdast/
├── mdxish/
├── plain/
├── render-mdxish/
├── compile.test.ts
├── custom-components-mdx.test.tsx
├── gemoji.test.ts
├── hast.test.ts
├── mdx.test.ts
├── mix.test.ts
├── owlmoji.test.ts
├── plain.test.ts
├── renderMdxish.test.tsx
├── run.test.tsx
├── stripComments.test.ts
└── tags.test.ts

After

__tests__/lib/
├── __snapshots__/
│   ├── owlmoji.test.ts.snap
│   └── stripComments.test.ts.snap
├── mdx/                                ← new
│   ├── __snapshots__/
│   │   └── mdx.test.ts.snap           ← regenerated
│   ├── exports/                        ← moved from lib/
│   ├── mdast/                          ← moved from lib/
│   ├── compile.test.ts                 ← moved from lib/
│   ├── custom-components.test.tsx      ← moved + renamed
│   ├── mdx.test.ts                     ← moved from lib/
│   └── run.test.tsx                    ← moved from lib/
├── mdxish/                             ← existing, received 4 files
│   ├── ...existing files...
│   ├── gemoji.test.ts                  ← moved from lib/
│   ├── hast.test.ts                    ← moved from lib/
│   ├── mix.test.ts                     ← moved from lib/
│   └── renderMdxish.test.tsx           ← moved from lib/
├── plain/                              ← existing, received 1 file
│   ├── ...existing files...
│   └── plain.test.ts                   ← moved from lib/
├── render-mdxish/                      ← unchanged
├── owlmoji.test.ts                     ← stayed (utility)
├── stripComments.test.ts               ← stayed (utility)
└── tags.test.ts                        ← stayed (utility)

🧬 QA & Testing

• Broken on production.
• Working in this PR app.