[Task]: Add flake8 lint rule against using assert_ mock methods

[diox]

Description

Python's mock without autospec will happily execute unknown methods without a warning, so it's generally best to avoid mock's special methods that return something truthy or perform the assertion directly like assert_called_with() - accidentally calling <mock>.asssert_called_with (typo asssert) would happily return without raising anything.

Checking call_args_list or call_count is more verbose/uglier but safer.

See https://engineeringblog.yelp.com/2015/02/assert_called_once-threat-or-menace.html on that topic (nowadays assert_called_once() does exist, but that wasn't the case back then)

We should have a flake8 plugin linting our code against that.

Acceptance Criteria

Milestones/checkpoints

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1. flake8 tests fail on assert_called_with and similar methods

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• If I have identified that the work is specific to a repository, I have removed "repository:addons-server" or "repository:addons-frontend"

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