Silence dead_code warning on AppBuilder::with_config

This function is not currently used but is likely to be useful in
the future, so this change silences the dead_code warning.

helix-term/tests/test/helpers.rs

@@ -286,6 +286,8 @@ impl AppBuilder {
         self
     }
 
+    // Remove this attribute once `with_config` is used in a test:
+    #[allow(dead_code)]
     pub fn with_config(mut self, config: Config) -> Self {
         self.config = config;
         self