Add integration and end-to-end tests for the Software Heritage stack

As we now have a full docker-compose environment enabling to run a complete Software Heritage instance, we should use it to implement integration and end-to-end tests.

For instance, one of those tests could consist in:

1. Start a Software Heritage instance, check all components are up
2. Create loading tasks for different types of software origin, check they get correctly executed
3. Check the metadata indexers have been executed once the loading tasks have succeed
4. Check if the ingested data can be accessed through the Web API and the browse UI.

This is a simple scenario and more test cases could be added.

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Migrated from T1618 (view on Phabricator)