Migrate dvcs loader tests code to pytest

They are hard to maintain (and even read) in their current state.

It's an entangled boil of inherited classes whose perimeter is conflated together (initialize svn repository, initialize loader with a given state, etc...).

It's complicated to the point of adding a new scenario about multiple visits loading scenario is not a simple task [1].

It'd be nice to disentangle this and also aligns with what other loaders are now using, pytest fixtures!

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Migrated from T2481 (view on Phabricator)