lib_protocol: use test stanza

Context

Use the tests stanza proto_alpha. I skipped lib_benchmark because it mixes tests with non-tests. The tests in lib_plugin were skipped accidentally, see commit message for details.

Also, the tests test_gas_properties.ml in lib_protocol/test was not run, it will be run now.

I've backported the changes, and I took the opportunity to fix the Invocation: fields of test headers that referred to proto_alpha.

Manually testing the MR

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.
• Select as Assignee the next person who should take action on that MR