cose: extract suite, move Alcotest.run to test.ml
Refactor test_cose.ml so it only exports a single 'suite' value with
prefixed test names (sign1: ..., header: ..., rfc9052: ..., hostile:
...). Move the Alcotest.run call to a new test.ml runner and rename
the dune (test (name ...)) accordingly (E600).