chore: improve miner simulator flakiness

[glevco]

Motivation

Due to the expected behavior of the StopAfterNMinedBlocks trigger, which is to stop the simulation after finding "at least N blocks", some tests were flaky. A new mechanism is introduced directly on the miner simulator to guarantee it stops producing blocks after exactly N blocks are found.

This new mechanism is used specifically in Feature Activation mining simulation tests, but any other tests that are found to be flaky because of this behavior, could be migrated to this alternative.

Acceptance Criteria

• Implement new pause_after_exactly() method for miner simulators
• Update Feature Activation mining simulation tests to use this new method

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

Merging #753 (6ee97e8) into master (4277cc2) will increase coverage by 0.04%.
Report is 6 commits behind head on master.
The diff coverage is 84.44%.

❗ Current head 6ee97e8 differs from pull request most recent head bd5311f. Consider uploading reports for the commit bd5311f to get more accurate results

@@            Coverage Diff             @@
##           master     #753      +/-   ##
==========================================
+ Coverage   84.62%   84.67%   +0.04%     
==========================================
  Files         253      252       -1     
  Lines       21708    21770      +62     
  Branches     2941     2946       +5     
==========================================
+ Hits        18371    18434      +63     
+ Misses       2702     2692      -10     
- Partials      635      644       +9     

Files Changed	Coverage Δ
hathor/simulator/trigger.py	100.00% <ø> (ø)
hathor/p2p/utils.py	73.87% <66.66%> (-1.40%)	⬇️
hathor/builder/builder.py	91.51% <80.00%> (-1.49%)	⬇️
hathor/builder/cli_builder.py	74.48% <100.00%> (+0.53%)	⬆️
hathor/manager.py	82.76% <100.00%> (+0.07%)	⬆️
hathor/mining/block_template.py	94.33% <100.00%> (+5.87%)	⬆️
hathor/p2p/resources/status.py	100.00% <100.00%> (ø)
hathor/p2p/states/ready.py	93.89% <100.00%> (-0.05%)	⬇️
hathor/simulator/miner/geometric_miner.py	100.00% <100.00%> (ø)
hathor/transaction/base_transaction.py	93.99% <100.00%> (+<0.01%)	⬆️

... and 4 files with indirect coverage changes

The base branch was changed.