Proto/test: update endorsement tests

Context

Some tests in test_endorsement are done at multiple places for various operation validation/application modes, sometimes all modes but not always. Motivated by !7815 (merged) which will bring even more differences between modes, the present MR refactors these tests to make it easier to:

• ensure that a given test is run on all modes, and
• adjust the expected outcome for one specific mode.

Manually testing the MR

CI

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• [N/A] Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• [N/A] 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