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 style: make clippy more strict #743,
• the tests are rewritten using a macro,
• the way how these functions react to empty input was changed.

Checklist:

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

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

Additional details and impacted files

@@            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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