Improve tests and labeling in FilesystemShell (#1305)

Additional `ls` command tests and better FilesystemShell error and label messages.

tests/commands/rm.rs

@@ -132,7 +132,7 @@ fn errors_if_attempting_to_delete_a_directory_with_content_without_recursive_fla
         );
 
         assert!(dirs.test().exists());
-        assert!(actual.contains("Cannot remove non-empty directory"));
+        assert!(actual.contains("cannot remove non-empty directory"));
     })
 }