Create Forc wallet configuration for tests

[sdankel]

In order to test CLI flow that interact with a user's wallet, we need some test utilities for creating a "test" wallet, starting a node with that wallet's accounts funded, and overriding the behavior of default_wallet_path to point to the test wallet when it's called from a test context, so as not to interfere with the user's wallet.

This will unlock the ability to write comprehensive end-to-end tests for forc-client.