change(ci): add acceptance test for getblocktemplate RPC in CI, and fix RPC bugs

[teor2345]

Motivation

We want to test the getblocktemplate RPC with a synced Zebra instance.
We tried this in PR #5653, but the RPC had some bugs, so we had to revert it.

Closes #5652.

Solution

• Re-apply PR change(tests): add acceptance test for getblocktemplate method in CI #5653 (do a revert of the revert)
• Fix bugs in the RPC method
  • An assert that was doing the wrong check, causing a panic
  • A fee that wasn't actually negated, causing a panic
• Fix coverage bugs in the test
  • The original RPC query is sent right after Zebra activates the mempool, when it is usually empty
  • Wait for transactions to download and verify in the test, then call the RPC again

Review

This is a high priority, because it stops us adding more bugs to the getblocktemplate RPC.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
• How do you know it works? Does it have tests?

Follow Up Work

• Manually test that each mining RPC works correctly #5686

[teor2345]

I have run this RPC manually thousands of times, and it seems to work (and it's very fast)

[codecov]

Codecov Report

Merging #5761 (7859d08) into main (f573365) will increase coverage by 0.04%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5761      +/-   ##
==========================================
+ Coverage   78.69%   78.73%   +0.04%     
==========================================
  Files         306      306              
  Lines       38687    38687              
==========================================
+ Hits        30445    30461      +16     
+ Misses       8242     8226      -16     

[arya2]

Looks good to me!