Context

Various improvements to lib_shell/test/test_prevalidation.ml in preparation for !6787 (merged), which features large changes to prevalidation.ml.

Manually testing the MR

dune exec src/lib_shell/test/main.exe -- -f test_prevalidation.ml

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).
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