Put generic test cases as TOML, not inside a custom macro

[lo48576]

Currently, generic test cases are stored as a native Rust value written using a macro.
This prevents auto-formatting and reduces the flexibility and maintainability of the test cases schema.

https://github.com/lo48576/iri-string/blob/bb71787bb920cb1bfdd2d455954b7a2e740a161a/tests/components/mod.rs

They are static data, so they should be stored on some well-known format such as TOML.
It requires additional dev-dependencies (only for testing), but is worth doing.