Port alcotests to alcotezts in cryptography source trees

Alcotests are deprecated but may still be used in cryptography-related source trees after the recent migration of these source trees to octez monorepo.

Progress:

• lib_polynomial: !7932 (merged)

• lib_bls12_381_polynomial: !8223 (merged)

• Decide if/how to reinstate privacy-team slow tests (perhaps as Tezt long tests)