Remove usage of empty signing keys in developer_tool

Stop using empty signing keys lists for writing unsigned metadata
as partially signed.
Update tests accordingly.

Signed-off-by: Teodora Sechkova <[email protected]>

Author: sechkova

tests/test_developer_tool.py

@@ -348,7 +348,9 @@ def test_write(self):
     name_backup = project.project_name
 
     # Write and reload.
-    self.assertRaises(securesystemslib.exceptions.Error, project.write)
+    self.assertRaises(tuf.exceptions.UnsignedMetadataError, project.write)
+    project.load_signing_key(project_private_key)
+    project('delegation').load_signing_key(delegation_private_key)
     project.write(write_partial=True)
 
     project = developer_tool.load_project(local_tmp)

tuf/developer_tool.py

@@ -479,9 +479,8 @@ def _generate_and_write_metadata(rolename, metadata_filename, write_partial,
   # written with write() / write_partial().
   # Increment the version number if this is the first partial write.
   if write_partial:
-    temp_signable = repo_lib.sign_metadata(metadata, [], metadata_filename,
-        repository_name)
-    temp_signable['signatures'].extend(roleinfo['signatures'])
+    temp_signable = repo_lib.sign_metadata(metadata, roleinfo['signing_keyids'],
+        metadata_filename, repository_name)
     status = tuf.sig.get_signature_status(temp_signable, rolename,
         repository_name)
     if len(status['good_sigs']) == 0: