tests: Always cleanup when exiting modify_env context manager

Previously, an exception would leave the environment changed.

tests/utils.py

@@ -54,7 +54,9 @@ def modify_env(**env):
     original_env = os.environ.copy()
     for k, v in env.items():
         os.environ[k] = v
+    try:
         yield
+    finally:
         for k in env:
             try:
                 os.environ[k] = original_env[k]