Remove ad hoc is_valid_for_signature method

Use the new generic is_public method.

Impact on generated cases: there are new HMAC test cases for SIGN_HASH. It
was a bug that these test cases were previously not generated.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/scripts/generate_psa_tests.py b/tests/scripts/generate_psa_tests.py
index ce15ac9..25bdf41 100755
--- a/tests/scripts/generate_psa_tests.py
+++ b/tests/scripts/generate_psa_tests.py
@@ -204,7 +204,7 @@
                 continue
                 # For public key we expect that key generation fails with
                 # INVALID_ARGUMENT. It is handled by KeyGenerate class.
-            if not kt.name.endswith('_PUBLIC_KEY'):
+            if not kt.is_public():
                 yield test_case_for_key_type_not_supported(
                     'generate', kt.expression, bits,
                     finish_family_dependencies(generate_dependencies, bits),
@@ -820,8 +820,10 @@
                 for key_type in sorted(alg_with_keys[alg]):
                     # The key types must be filtered to fit the specific usage flag.
                     kt = crypto_knowledge.KeyType(key_type)
-                    if kt.is_valid_for_signature(usage):
-                        yield self.keys_for_implicit_usage(usage, alg, kt)
+                    if kt.is_public() and '_SIGN_' in usage:
+                        # Can't sign with a public key
+                        continue
+                    yield self.keys_for_implicit_usage(usage, alg, kt)
 
     def generate_all_keys(self) -> Iterator[StorageTestData]:
         yield from super().generate_all_keys()