Fix non idiomatic function names warnings in the std lib.

[tritao]

warning
   --> sway-lib-std/src/primitive_conversions/u64.sw:189:4
    |
187 |
188 | #[test]
189 | fn test_u64_try_from_U128() {
|    ---------------------- Function name "test_u64_try_from_U128"
is not idiomatic. Function names should be snake_case, like
"test_u64_try_from_u128".
190 |     use ::assert::assert;
    |

Description

Checklist

• I have linked to any relevant issues.
• I have commented my code, particularly in hard-to-understand areas.
• I have updated the documentation where relevant (API docs, the reference, and the Sway book).
  • If my change requires substantial documentation changes, I have requested support from the DevRel team
• I have added tests that prove my fix is effective or that my feature works.
• I have added (or requested a maintainer to add) the necessary Breaking* or New Feature labels where relevant.
• I have done my best to ensure that my PR adheres to the Fuel Labs Code Review Standards.
• I have requested a review from the relevant team or maintainers.

[IGI-111]

These are tests, so it's fine, but if these were actually part of the std interface I'd want to maintain the capitalization. We may have builtin u128 at some point in the future.