Introduce test targets, get rid of test compilation being fanout among Scarb & cairo-test

[mkaput]

We have an incident ⚠ All of: Scarb, Protostar and cairo-test-runner are manually constructing the RootDatabase and compiling tests on their own. The reason for this was to quickly deliver working software. We should eventually fix that!

The plan is:

1. Let Scarb define a built-in [[test]] target.
2. Decide between all 3 projects what kinds of artifacts should this target produce. It should definitely include built, loadable Sierra code & some metadata files at least defining test functions paths.
3. Migrate Protostar & cairo-test-runner to expect tests to be compiled externally by Scarb and only execute these artifacts.

[mkaput]

Q: Perhaps the cairo-runner could also make a use of this target? @4rgorok

[maciektr]

Please do not forget to support /tests dir in projects structure https://github.com/starkware-libs/cairo/issues/3777 https://github.com/foundry-rs/starknet-foundry/issues/289

[maciektr]

Closing, as Starknet Foundry is migrated to procedural macros now 🥳 https://github.com/foundry-rs/starknet-foundry/tree/master/crates/snforge-scarb-plugin