p2p/tezt: add flaky tag to tests using forked processes

Context

As describe in #5903, the tests p2p socket nack, p2p socket self peer id and p2p socket self connection message are flaky. The previous MR !9116 (merged) do not prevent these tests to keep failing in the CI.

Add flaky tag to tests until #5903 is fixed.

This MR depends of !9158 (merged).

Manually testing the MR

Check that the command below list the tests p2p socket nack, p2p socket self peer id and p2p socket self connection message.

dune exec tezt/tests/main.exe --  --list flaky

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