Tezt: Revert change from kill to terminate for Agnostic_baker

What

Closes #7786 (closed).

Why

In !16833 (merged), a lot of Baker occurrences in tezt tests got replaced to Agnostic_baker to better consolidate the product. However, due to some unknown reasons, some tests failed when replacing Baker.kill with Agnostic_baker.kill, but worked when changing it to Agnostic_baker.terminate.

In this MR, I revert those changes back to Agnostic_baker.kill, and from what I see, everything works normally with the affected tests. So, it must have been the case that the agnostic baker was not handling well the killing process before, but does it better now (enough to not be spotted by the tests).

How

Manually testing the MR

CI green, but more precisely, the affected tests are:

dune exec tezt/tests/main.exe -- --file baker_test.ml -v --title 'Alpha: Baker check branch in consensus operations'

dune exec tezt/tests/main.exe -- --file nonce_seed_revelation.ml -v --title 'Alpha: Baking nonce format migration'

dune exec tezt/tests/main.exe -- --file synchronisation_heuristic.ml -v --title 'Alpha: bootstrap: test threshold one'

dune exec tezt/tests/main.exe -- --file synchronisation_heuristic.ml -v --title 'Alpha: bootstrap: test threshold two'

(of course you can change Alpha: with any protocol you want, or test the files as a whole)

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