Add CI test for manually maintained tests.toml files

See [here](https://github.com/exercism/rust/pull/2050#issuecomment-2708942397) for the original idea.

Basically, some exercises don't lend themselves to generating tests based on a template. We still want to be sure that the `tests.toml` produced by `configlet` matches a manually maintained test file. For this purpose, we could have a test that parses the `tests.toml` (using `toml` crate) and checks that for each test, a function named `make_ident(description)` exists in the rust test file.

Some test names may differ, in which case `rust_fn` in `tests.toml` can be used to override `make_ident(description)`.