Test scaffolding crate, and integration tests for the Signet Node.

This library contains the following:

• SignetTestContext - A running test node with alloy providers and the ability to send transactions, query state, and mine blocks.
• run_test - A helper function to run async tests with a fresh SignetTestContext.
• rpc_test - A wrapper around run_test for testing RPC methods.
• Assorted test utilities and constants

The tests/ directory contains a number of integration tests for the Signet Node, using the above scaffolding.