Move fixture generation to contrib and add test

[guillep2k]

As suggested in #10247 (review), I've moved the fixture generation to contrib:

go run -tags 'sqlite sqlite_unlock_notify' contrib/fixtures/fixture_generation.go

I've also added a unit test for checking if the derivative fixtures (currently access.yml) match the expected generation.

It looks more "natural", but it's a lot more of code. I have mixed feelings of whether this is the best way or the one we've just merged. I'll leave the decision to you guys.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@b2f04f4). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master   #10277   +/-   ##
=========================================
  Coverage          ?   43.68%           
=========================================
  Files             ?      585           
  Lines             ?    80988           
  Branches          ?        0           
=========================================
  Hits              ?    35380           
  Misses            ?    41227           
  Partials          ?     4381

Impacted Files	Coverage Δ
models/fixture_generation.go	65.38% <100%> (ø)

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