tests: Small refactor of a test

Test was supposed to test a threshold that is higher than number of
signatures, but it actually was just using completely unsigned metadata.

This still doesn't test the case where _trusted_ metadata defines a
threshold that new metadata does not reach: only the case where new
metadata defines threshold that it does not meet (this case is covered
in updater tests though).

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

Author: Jussi Kukkonen

tests/test_trusted_metadata_set.py

@@ -240,12 +240,14 @@ def root_new_version_modifier(root: Root) -> None:
         self.trusted_set.update_root(root)
 
     def test_update_root_new_root_fail_threshold_verification(self) -> None:
-        # new_root data with threshold which cannot be verified.
-        root = Metadata.from_bytes(self.metadata[Root.type])
-        # remove root role keyids representing root signatures
-        root.signed.roles[Root.type].keyids.clear()
+        # Increase threshold in new root, do not add enough keys
+        def root_threshold_bump(root: Root) -> None:
+            root.version += 1
+            root.roles[Root.type].threshold += 1
+
+        root = self.modify_metadata(Root.type, root_threshold_bump)
         with self.assertRaises(exceptions.UnsignedMetadataError):
-            self.trusted_set.update_root(root.to_bytes())
+            self.trusted_set.update_root(root)
 
     def test_update_root_new_root_ver_same_as_trusted_root_ver(self) -> None:
         with self.assertRaises(exceptions.BadVersionNumberError):