[TST] Add submodule checkout step to CI test suite

[surchs]

When our CI suite runs on GH, we want to make use of the new SPARQL test graph introduced in #357. However, this new graph depends on two new submodules (also introduced in #357) to be available. We therefore have to

• Update the CI test workflow to initialize the new submodules
• Launch the local test graph before the tests are run in the CI
• Ensure all tests are run, including the integration tests (the ones that do not mock the graph)

Originally posted by @alyssadai in #357 (comment)