tests: Add basic smoke tests for Key

python-tuf has an extensive test suite for Metadata in general:
* I don't think python-tuf should stop testing Key just because it's now in
  securesystemslib
* Maybe it's not necessary to copy e.g. all serialization tests here?

Still, added a few basic tests for Key. Key is also used in the Signer tests.

tests/test_signer.py

@@ -27,6 +27,45 @@
 )
 
 
+class TestKey(unittest.TestCase):
+    """Key tests. See many more tests in python-tuf test suite"""
+
+    def test_key_from_to_dict(self):
+        keydict = {
+            "keytype": "rsa",
+            "scheme": "rsassa-pss-sha256",
+            "extra": "somedata",
+            "keyval": {
+                "public": "pubkeyval",
+                "foo": "bar",
+            },
+        }
+
+        key = Key.from_dict("aa", copy.deepcopy(keydict))
+        self.assertDictEqual(keydict, key.to_dict())
+
+    def test_key_is_verified(self):
+        sigdict = {
+            "keyid": "e33221e745d40465d1efc0215d6db83e5fdb83ea16e1fb894d09d6d96c456f3b",
+            "sig": "3fc91f5411a567d6a7f28b7fbb9ba6d60b1e2a1b64d8af0b119650015d86bb5a55e57c0e2c995a9b4a332b8f435703e934c0e6ce69fe6674a8ce68719394a40b",
+        }
+        keydict = {
+            "keytype": "ed25519",
+            "scheme": "ed25519",
+            "keyval": {
+                "public": "8ae43d22b8e0fbf4a48fa3490d31b4d389114f5dc1039c918f075427f4100759",
+            },
+        }
+        key = Key.from_dict(
+            "e33221e745d40465d1efc0215d6db83e5fdb83ea16e1fb894d09d6d96c456f3b",
+            keydict,
+        )
+        sig = Signature.from_dict(sigdict)
+
+        self.assertTrue(key.is_verified(sig, b"DATA"))
+        self.assertFalse(key.is_verified(sig, b"NOTDATA"))
+
+
 class TestSigner(unittest.TestCase):
     """Test Signer and SSlibSigner functionality"""