Fix dev check rejecting exercises with CRLF line endings

[SandmeyerX]

Problem

On Windows (or whenever an editor saves an exercise with CRLF line endings),
rustlings dev check fails with:

The file `exercises/XX/xxx.rs` doesn't contain any tests.

even though the exercise clearly contains #[test] functions.

Solution

Normalize CRLF to LF when checking whether an exercise contains tests, and add some unit tests.

[SandmeyerX]

Close #2424

[senekor]

Can we just omit the line ending? i.e. just check for #[test] instead of #[test]\n. Not sure, haven't thought too much about it.

[SandmeyerX]

Can we just omit the line ending? i.e. just check for #[test] instead of #[test]\n. Not sure, haven't thought too much about it.

@senekor Thanks for the review! I'd lean toward keeping the \n check - reasoning:

The trailing newline was actually added deliberately in e74f2a4 — the check originally was a bare contains("#[test]"), and I suppose the \n was introduced to avoid false positives where #[test] merely appears in prose, e.g.:

// TODO: write a #[test] function here

With a bare #[test] match, that comment would make the exercise count as "containing tests".

Maybe "More means better".

[senekor]

I see, makes sense. Thanks for digging. Looks good to me.