Proto/test_aggregate: add unit testing for preattestations_aggregate protocol implementation

What

Rename tests, add helpers, and add unit testing for the following behaviours :

• a valid preattestations_aggregate with a single delegate is correctly validated
• a valid preattestations_aggregate with multiple delegates is correctly validated
• a preattestations_aggregate with an empty committee is invalid
• a preattestations_aggregate with an invalid signature is invalid
• a preattestations_aggregate with a non-BLS delegate is invalid

Why

How

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, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR