Improve Test Documentation

[0xBradock]

Hello,

Concentrate test examples in a dedicated test/<test-case>/ in the root of this repo.
Suggested list of tests to implement.

Request / Response:

• response-codes
• query-parameters
• json-body
• x-www-form-urlencoded
• static-file
• jwt
• middleware

Internals:

• application-state
• custom-response
• error-handling
• threads

Protocols:

• stream
• multipart-stream
• websocket
• http/2
• https
• protobuf
• sse

External integration:

• mock-db
• sqlx
• diesel-rs

If you think of other scenarios I can add them to the list above.