style: cleanup `run_length_encoding.rs`

[vil02]

Pull Request Template

Description

This PR cleanups the run_length_encoding.rs:

• the string_to_string is removed from the suppression list - continuation of #743,
• the tests are rewritten using a macro,
• the way how these functions react to empty input was changed.

Checklist:

• [x] I ran bellow commands using the latest version of rust nightly.
• [x] I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• [x] I ran cargo fmt just before my last commit.
• [x] I ran cargo test just before my last commit and all tests passed.
• [x] I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).
• [x] I added my algorithm to DIRECTORY.md with the correct link.
• [x] I checked COUNTRIBUTING.md and my code follows its guidelines.

Please make sure that if there is a test that takes too long to run ( > 300ms), you #[ignore] that or try to optimize your code or make the test easier to run. We have this rule because we have hundreds of tests to run; If each one of them took 300ms, we would have to wait for a long time.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.01%. Comparing base (2544d28) to head (32895ca).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #744 +/- ## ========================================== - Coverage 95.02% 95.01% -0.01% ========================================== Files 303 303 Lines 22563 22526 -37 ========================================== - Hits 21440 21403 -37 Misses 1123 1123 ```

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