message: fix a test for std with backtraces
diff: @@ -655,9 +655,13 @@ mod tests {
  fn implements_debug() {
  let error: StdError = StdError::from(OverflowError::new(OverflowOperation::Sub, 3, 5));
  let embedded = format!("Debug: {:?}", error);
  + #[cfg(not(feature = "backtraces"))]
  + let expected = r#"Debug: Overflow { source: OverflowError { operation: Sub, operand1: "3", operand2: "5" } }"#;
  + #[cfg(feature = "backtraces")]
  + let expected = r#"Debug: Overflow { source: OverflowError { operation: Sub, operand1: "3", operand2: "5" }, backtrace: <disabled> }"#;
  assert_eq!(
  embedded,
  - r#"Debug: Overflow { source: OverflowError { operation: Sub, operand1: "3", operand2: "5" } }"#
  + expected
  );
  }
  