Add new tests

[davfsa]

Summary

Checklist

• I have run nox and all the pipelines have passed.
• I have made unittests according to the code I have added/modified/deleted.

Related issues

#47

[codecov]

Codecov Report

Merging #80 into master will increase coverage by 0.55%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master      #80      +/-   ##
==========================================
+ Coverage   79.77%   80.33%   +0.55%     
==========================================
  Files          78       78              
  Lines        9711     9712       +1     
  Branches     1238     1238              
==========================================
+ Hits         7747     7802      +55     
+ Misses       1908     1854      -54     
  Partials       56       56              

Impacted Files	Coverage Δ
hikari/api/event_dispatcher.py	100.00% <ø> (ø)
hikari/impl/event_manager_base.py	65.00% <85.71%> (+39.10%)	⬆️

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[FasterSpeeding]

lgtm