[EXTERNAL] fix(rust-piscine): fix checkpoint subjects

[sadiqui]

1. check_user_name

• Use Rust underscore style in User field.
• Explicitly specify enum type AccessLevel.
• check_user_name takes references to User.
• Tests expect "ERROR" to be entirely uppercase.

https://github.com/01-edu/rust-tests/blob/a38b2f91f5e3e38920de07d5c6bd8d6cec20623e/tests/check_user_name_test/src/main.rs#L21-L24

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2. get_document_id

• Structures are already offered.
• Highlight the needed task.

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3. previous_prime

• Tests don't expect prime argument to be returned.

https://github.com/01-edu/rust-tests/blob/a38b2f91f5e3e38920de07d5c6bd8d6cec20623e/tests/previousprime_test/src/main.rs#L15-L19

[pedrodesu]

Hello @sadiqui! Thank you for the extensive PR. I'd like to address all the changes you requested:

• The first and third ammends on check_user_name totally make sense. They should be changed. The second one isn't much of a necessity, as the 'expected functions and structures' section already states we're borrowing the reference, not moving. Nevertheless, this change can stay to remove ambiguity.
• The change on previousprime was nullified, as the problem was already identified and fixed in a newer version of the repo.
• The change on get_document_id doesn't seem necessary either, it's a bit of a matter of interpretation. You're still manually creating the structures/functions/what not, the 'expected' section ideally serves more as a hard verification for the student to ensure they have the minimal structure that the exercise asks for, and that they have the needed technicalities to pass the subject (for example, whether we're passing an instance of a method by value or reference). This isn't a big deal at all, it's just a clarification for the future. As such I'll return this line to its original state. All the remaining changes will stay and be merged, however.

Thank you sir. Please let me know if you'd like to comment on something else.