fix(events): fix bugs in the event stream and improve tests

[glevco]

Acceptance Criteria

• Add missing docstrings and typings
• Fix parsing bug in EventData and add tests for it
• Fix edge cases bugs in EventWebsocketProtocol and add tests for them
• Implement BaseEventSimulationTester to help writing event queue feature tests, and new tests using it

[codecov]

Codecov Report

Merging #647 (af933bc) into master (7ac143f) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

❗ Current head af933bc differs from pull request most recent head 768e9d2. Consider uploading reports for the commit 768e9d2 to get more accurate results

@@            Coverage Diff             @@
##           master     #647      +/-   ##
==========================================
- Coverage   83.88%   83.85%   -0.04%     
==========================================
  Files         247      247              
  Lines       20376    20378       +2     
  Branches     2748     2748              
==========================================
- Hits        17093    17088       -5     
- Misses       2709     2714       +5     
- Partials      574      576       +2     

Impacted Files	Coverage Δ
hathor/event/model/base_event.py	100.00% <ø> (ø)
hathor/event/model/event_type.py	100.00% <ø> (ø)
hathor/event/websocket/factory.py	92.85% <ø> (ø)
hathor/builder/builder.py	92.99% <100.00%> (+0.01%)	⬆️
hathor/event/event_manager.py	91.74% <100.00%> (+0.07%)	⬆️
hathor/event/model/event_data.py	100.00% <100.00%> (ø)
hathor/event/websocket/protocol.py	95.83% <100.00%> (ø)

... and 6 files with indirect coverage changes

The base branch was changed.

[jansegre]

Just a small note, we normally use fix instead of bugfix for both branch prefix and the commit message.