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

[Eikix]

In an effort of having fully documented code, we need the way we run tests for a single file to be documented.

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

[gdnathan]

I'd like to contribute on this issue