Restructure tests for multi-staged builds (if necessary)

[evrardjp]

Currently the tests are in the root folder. They are easily discoverable, but it's gonna be a mess if we do different kinds of testing.

I am suggesting here to create multiple folders based on test cases:

• tests/single/test_<language>.py can take care of the simple builds, for those not doing multi-staged builds
• tests/multistaged/test_<language>.py to test multi staged builds
• tests/pack/test_<buildpack>.py to ensure paketo/buildpacks.io are working as expected with our base images.

For this, we will have to:

• rewire tox to point to the right tests
• refactor some tests: extract them from tests_ and split them into the right folders
• Create multi staged containers in obs
• Modify data structures in matryoshka_tester/data.py to add the newly built containers. If data.py become more convoluted, we might want to add convenience datastructures. We need to adapt existing tests to new structure.

[evrardjp]

This can stay open, even if we moved to multi-staged. We'll see if we need re-organisation of data later.