Add an empty NetworkParameters struct to Network::Testnet

[arya2]

Motivation

Zebra currently hard-codes some consensus parameters that may need to be changed for testing.

Depends-On: #8325, #8326

Possible Design

• Add an empty and optional TestnetParameters struct in Network::Testnet
• Add a Network::new_testnet() method that returns Network::Testnet { ..default_testnet_params }
  • Mass replace any remaining instances of Testnet in tests with Network::new_testnet()

Example (with fields that shouldn't be added yet):

Testnet(Arc<NetworkParameters { 
   cache_name,
   genesis_hash,
   network_id,
   default_port,
   activation_heights
>)

[AloeareV]

1. refactor tests to use `is_mainnet()` to look up test vectors using new functions in `zebra-test`, rather than matching on `Testnet`. This should be quick and easy to approve since it's only test code, and there are only mainnet and default public testnet test vectors. (No custom testnet vectors.)

If I'm going to implement this, is the intent here to replace the match network blocks with if network.is_mainnet() blocks? New functions in zebra-test like some kind of fn test_vector_for_network that gets offloaded to?

[arya2]

If I'm going to implement this, is the intent here to replace the match network blocks with if network.is_mainnet() blocks?

We want to do this to minimize the changes in the following PRs where the Testnet variant is modified, but this issue is for adding an empty struct in the Testnet variant of Network. Replacing match network blocks with if network.is_mainnet() in tests should precede this issue as an early step in #7845, but we're still planning #7845 so there isn't an open issue for this yet.

We decided PR #7924 was too large to test or review accurately, and because this is a high-risk area of the code, we want to divide the changes across multiple PRs, where each PR adds one configurable parameter and Network method with unit tests for its changes.

We also discussed #1135 around the time that #7924 was opened, and decided to add any configurable parameters to the Testnet variant instead of adding a new variant or refactoring Network into a trait to reduce scope.

[arya2]

I split the "Multiple PRs" section of this issue into a separate issues in #7845.