Split `unit:alltest` in the CI into proto / non-proto tests

Context

Fixes #1158 (closed). Implement @vivienpe suggestion here: #1158 (comment 525689175)

A good approach could be to extract unit test of each protocol into a dedicated CI task.

Manually testing the MR

Check the CI. Make sure no tests have been lost:

$ diff <(grep test_wrapper .gitlab/ci/unittest.yml | sort) <(grep test_wrapper ../master/.gitlab/ci/unittest.yml | sort)

The first commit contains a refactoring of the artifact configuration for these jobs. It should be a noop.

Results

Pipeline visualization before (click to expand): https://gitlab.com/nomadic-labs/tezos/-/pipelines/285740628

Pipeline visualization after (click to expand): After: https://gitlab.com/nomadic-labs/tezos/-/pipelines/285737597

For the given comparison examples, the test stage has a gained wall-time of 48-26=22 minutes.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.