Assert the column is unknown.

did you mean in error messages appear when `get`ing unknown columns. Here we know the column does not exist so we check the exact error message.

tests/command_config_test.rs

@@ -108,8 +108,7 @@ fn removes_configuration_value() {
             dirs.config_path()
         );
 
-        println!("{}", actual);
-        assert!(actual.contains("did you mean"));
+        assert!(actual.contains("Unknown column"));
     });
 
     h::delete_file_at(nu::config_path().unwrap().join("test_5.toml"));