Add testnet integration test for HelloWorld

[gator-boi]

Objective

Add an integration test that allows Hello World apps to speak to each other on testnet.

Definition of Done

In a new directory (probably called testnet in the root of the repository), build out how the tests should be organized and add a test that performs sendMessage and receiveMessage from each EVM, Solana and Aptos contract.

Please be mindful of other apps from new networks joining the test. For example, if a Hello World Algorand app is written, each app should interact with the other two apps (Algorand <> EVM and Solana, Solana <> Algorand and EVM, etc). We prefer the tests to be exhaustive, so each network's app interacting with all other networks' apps.