Test dispute with pending contributions

[er1c-cartman]

Summary

• add regression coverage for a dispute raised while the active round still has pending contributions
• verify already-recorded contributions and total_contributed are preserved while the group is disputed
• verify resolving the dispute returns the group to active status and the remaining member can contribute to complete the round

Fixes #75.

Tests

• cargo test --package sorosave
• cargo fmt --check
• cargo clippy --package sorosave --all-targets -- -D warnings
• git diff --check