refactor: simplify redundant string conversions

[MamunC0der]

Removes unnecessary .to_string().as_str() patterns that cause redundant allocations.

• Simplified string conversions in colored_printer.rs and test_utils.rs
• No functional changes, optimization only

[reviewable-StarkWare]

This change is 

[orizi]

@orizi reviewed 2 of 2 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @MamunC0der)