Tezt: fix flaky external validator test

Context

Actually wait for the external validator to die before trying to bake, otherwise the node thinks its dead when it is in fact dying (closing fd probably).

fixes #5467 (closed)

Manually testing the MR

dune exec tezt/tests/main.exe -- --file external_validation.ml --verbose

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.
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