Reusable mock for events

[einarmo]

Migrates the tests for events to use a proper events mock.

There is a workaround here for an old, old possibly bug in the events API, but it's a little fragile, and not really that important to test anymore.

[codecov]

Codecov Report

❌ Patch coverage is 68.71166% with 51 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.54%. Comparing base (13177ba) to head (8439978).

Files with missing lines	Patch %	Lines
Cognite.Testing/Mock/Events.cs	68.75%	41 Missing and 4 partials ⚠️
Cognite.Testing/Mock/MockUtils.cs	70.58%	3 Missing and 2 partials ⚠️
Cognite.Testing/Mock/TimeSeries.cs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #602      +/-   ##
==========================================
- Coverage   77.75%   77.54%   -0.22%     
==========================================
  Files         136      138       +2     
  Lines       13174    13319     +145     
  Branches     1974     1993      +19     
==========================================
+ Hits        10244    10328      +84     
- Misses       2105     2164      +59     
- Partials      825      827       +2     

Files with missing lines	Coverage Δ
Cognite.Testing/Mock/TimeSeries.cs	75.42% <50.00%> (+0.03%)	⬆️
Cognite.Testing/Mock/MockUtils.cs	70.58% <70.58%> (ø)
Cognite.Testing/Mock/Events.cs	68.75% <68.75%> (ø)

... and 4 files with indirect coverage changes

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