Fix mypy assignment error

Fix:

```
lib/tool_shed/test/functional/test_shed_tools.py:30: error: Incompatible types
in assignment (expression has type "BuildSearchIndexResponse", variable has type
"ToolSearchResults")  [assignment]
            response = populator.reindex()
                       ^
lib/tool_shed/test/functional/test_shed_tools.py:31: error: Incompatible types
in assignment (expression has type "BuildSearchIndexResponse", variable has type
"ToolSearchResults")  [assignment]
            response = populator.reindex()
                       ^
```

lib/tool_shed/test/functional/test_shed_tools.py

@@ -27,8 +27,8 @@ def test_tool_search(self):
             assert tool_search_hit.tool.name == "Compute"
 
         # ensure re-index doesn't modify number of hits (regression of an issue pre-Fall 2022)
-        response = populator.reindex()
-        response = populator.reindex()
+        populator.reindex()
+        populator.reindex()
 
         response = populator.tool_search_query("Compute")
         new_hit_count = len(response.hits)