Feature/libmagic dependency injection

[conitrade-as]

This change set adds the magic description (as a payload) and mime type (as a header) to the classifier result.

Testing has been completely reworked and uses dependency injection to make libmagic results stable across various platforms.

[msm-code]

Thanks!

I feel like there's a lot of duplication, and I'm a bit concerned by the big number of new lines of code. For example, this fragment:

m = MagicMock()
m.side_effect = [
    ', '.join([
        'PDF document',
        'version 1.4',
    ]),
    'application/pdf',
]
self.karton = Classifier(
    magic=m, config=self.config, backend=self.backend
)

with a good helper method could be shortened to:

self.karton = mock_classifier(['PDF document', 'version 1.4'], "application/pdf")

And this fragment:

resource = Resource('file.pdf', b'feeddecaf\n', sha256='sha256')
task = Task({
    'type': 'sample',
    'kind': 'raw',
})
task.add_payload('sample', resource)

res = self.run_task(task)

could be shortened to:

res = classify("file.pdf")

this will add up over all the repeated invocations. Do you think it makes sense?

Also do you mind running black and flake8 on your code? Looks like CI doesn't check tests/ directory, but we like the consistency black enforces.

[conitrade-as]

Let me clean up the code just a bit more. As there seem to be some issues after merging in changes from @msm-code .

[msm-code]

Tests still feel a bit repetitive, but I think they're good - and I like the dependency injection approach here. LGTM.