test: Share our testing fixtures efficiently

[akhercha]

Feature Request

Current behavior

We currently have a lot of duplicated test code (fixtures mainly) between our tests for pragma-sdk, price-pusher and vrf-listener and checkpoint-setter.

Expected behavior

We want to create a new pragma-testing poetry package that will be a pytest plugin holding every fixtures that can be shared across tests.
The different packages will then just import this package and use the fixtures.

EDIT: Creating a new pragma-testing package won't be possible as it will create a lot of circular dependencies (the fixtures that we want to use uses pragma-sdk).

Instead, we want to organize our fixtures better directly in the pragma-sdk package and allows the others packages to just import them as a pytest plugin.

Improvements

• Use testcontainer to start devnet-rs (image)
• Use the new DevnetClient from starknet.py