Unify forking network naming across Test Framework & Emulator

Currently, we recommend using custom networks for forking against the emulator, such that developers can have both a regular `mainnet` and a forked `fork-mainnet` network (or similar).

However, support was only added to the Flow Emulator initially.  To deliver an equivalent DevEx, and reduce friction, we should add custom network support to the Test Framework fork mode as well.