test: add five-member group lifecycle coverage

[Scottcjn]

Summary

• add a 5-member end-to-end lifecycle test for the Sorosave contract
• verify the payout order covers each member exactly once across 5 rounds
• assert each round completes with the expected contribution total and final balances return to the starting value

Testing

• cargo test

Closes #32

[a638011]

Closing: This bounty has no specified dollar amount and does not meet my criteria (USD > $1). Apologies for the noise.

[a638011]

Withdrawing this PR. This bounty has no specified dollar amount and does not meet my criteria (USD > $1). Apologies for the noise.