[Feature] Use a typed storage key enum for evil_token test module

[collinsezedike]

Summary

The evil_token test module in contracts/tholos/src/test.rs uses ad hoc symbol_short!("tholos"), symbol_short!("target"), and symbol_short!("bal") string keyed storage instead of a DataKey style enum, unlike the main contract's own well structured storage key pattern.

Scope

Test only code, low priority. Introduce a small enum for EvilToken's storage keys for consistency with the main contract's conventions.