lib_bls12_381_polynomial/tezt: use [Format] to print seed

Context

Using Format instead of Printf in this alcotezt lets Tezt handle the output, and will hide the seed unless --verbose is given. That is, less unwanted "Random seed: 12312312" in the output.

Manually testing the MR

Try dune exec src/lib_bls12_381_polynomial/test/main.exe with/without --verbose, before and after this patch.

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).
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