Crypto/BLS: add more unit tests

This MR adds more tests for BLS signatures:

• checks signature or proof verification against test vectors
• checks if pippenger and affine_add_bulk use complete formulas (they handle repeated points and a point at infinity correctly)
• checks if pop_prove/pop_verify works correctly with an override public key
• checks if pop_verify does not accept proof with Pop signature ciphersuite
• checks if verify does not accept signature with Aug signature ciphersuite

Manually testing the MR

dune exec src/lib_bls12_381_signature/test/main.exe -- --file test_signature.ml
dune exec src/lib_bls12_381/test/main.exe -- --file test_g1.ml
dune exec src/lib_bls12_381/test/main.exe -- --file test_g2.ml

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