test(contracts): add Soroban testing utilities and refactor tests (#115)

[edehvictor]

Summary

This PR abstracts repetitive Soroban testing boilerplate (Env creation, contract registration, mock auth, and address generation) into a reusable TestContext utility to improve test readability and developer experience.

Fixes #115

Changes Made

• Created src/test_utils.rs containing the TestContext struct and helper methods.
• Guarded the new module behind #[cfg(test)] in src/lib.rs to ensure zero bloat to the production WASM.
• Refactored the register_offering_emits_exact_event test in src/test.rs to act as a reference implementation for the new pattern, successfully reducing boilerplate.

How to Test

1. Pull this branch.
2. Run cargo test to verify that all tests remain green and the newly refactored test executes perfectly using the shared context.

[edehvictor]

Gm @thlpkee20-wq, kindly review