Add a contextual validity test for the getblocktemplate method

[arya2]

Motivation

A contextual validity check was added to the getblocktemplate RPC method in #5630.

While this would needlessly hurt performance in production, it's useful for testing that the getblocktemplate response will be contextually valid and accepted if submitted with a valid solution before another block is committed.

Designs

• See change(rpc): check contextual validity of getblocktemplate response #5630
  • Add #[cfg(test)] to the new code so that it will only run when Zebra is built for testing
• Sync Zebra to the tip
• Call getblocktemplate RPC with test configuration
  • Convert the getblocktemplate response into a prepared block with an invalid/empty solution
  • Apply all of the contextual validation logic as a speculative check to ensure that the block template is contextually valid

[arya2]

Closing this for now in favour of #5685