Lib_bls12_381_signature/tests: fix tests declaration

In src/lib_bls12_381_signature/test/test_aggregated_signature.ml two modules with test functions are instantiations of the same functor MakeAggregatedSignatureTest. This MR fixes the test declarations so that test cases from both these modules are run instead of running those from one module twice.

Manually testing the MR

dune build @src/lib_bls12_381_signature/runtest

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