Doc - Describe `list_test_entrypoints` using Rustdoc comments (three slashes: ///)

[Eikix]

In an effort of having fully documented code, we need the way we list entry points in a Cairo repository to be documented.

In src/cli/commands/test, please proceed to comment above the fn list_test_entrypoints declaration in an equivalent way as pub fn compile is documented.

[gdnathan]

I'd like to contribute on this issue