tests: Don't use os.path.join() for URLS

The test has issues like this alsready but let's not add more...

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

Author: Jussi Kukkonen

tests/test_updater_rework.py

@@ -123,8 +123,8 @@ def setUp(self):
     # directory copied from the original repository files.
     tuf.settings.repositories_directory = self.client_directory
 
-    metadata_url = os.path.join(url_prefix, 'metadata/')
-    targets_url = os.path.join(url_prefix, 'targets/')
+    metadata_url = f"{url_prefix}/metadata/"
+    targets_url = f"{url_prefix}/targets/"
     # Creating a repository instance.  The test cases will use this client
     # updater to refresh metadata, fetch target files, etc.
     self.repository_updater = updater.Updater(self.repository_name,