fix(rpc): Mine standard and minimum difficulty blocks on testnet

[teor2345]

Motivation

In PR #5659, I added a check for testnet minimum difficulty blocks. But it accidentally made all the mined blocks on testnet into minimum difficulty blocks.

Depends-On: #5761

Solution

Check if the current time is close to the minimum difficulty time (2 target block intervals). If it is, use a minimum difficulty block. If there is plenty of time, use a standard difficulty block.

Split the testnet adjustment into its own function.

Cleanup some names and constants.

Review

This is a routine bug fix, @oxarbitrage might want to review it.

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?

[codecov]

Codecov Report

Merging #5747 (e26c1c4) into main (afdb3a7) will decrease coverage by 0.08%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5747      +/-   ##
==========================================
- Coverage   78.79%   78.71%   -0.09%     
==========================================
  Files         306      306              
  Lines       38702    38702              
==========================================
- Hits        30496    30463      -33     
- Misses       8206     8239      +33     

[teor2345]

Failed due to #5384 😭

[teor2345]

@Mergifyio update

[mergify]

update

☑️ Nothing to do

• #commits-behind>0 [:pushpin: update requirement]
• -closed [:pushpin: update requirement]

[arya2]

Looks good!