Add integration tests against Stellar testnet

[big14way]

Write integration tests that interact with a deployed contract on testnet.

Acceptance Criteria:

• Set up test suite that uses real testnet RPC
• Test full lifecycle: create group, join, contribute, distribute
• Use friendbot for test account funding
• Skip tests in CI unless TESTNET_SECRET_KEY is set
• Add test timeout handling for slow network

Files: new file tests/integration/, package.json

[lustsazeus-lab]

Great acceptance criteria. For stability on Stellar testnet, it helps to wrap friendbot/account-creation in retry-with-jitter and mark flaky network failures separately from contract logic assertions.\n\nA small fixture helper for funded test accounts will keep lifecycle tests deterministic and easier to extend.