message: tests: fix lifetime warning
  Apparently we were hitting a bug in borrowck that was fixed, but now
  emits a warning. It doesn't appear necessary for us to return a borrow,
  so we just copy.
diff: @@ -35,7 +35,9 @@ replace!(no_expand2, replace,
  r"(\S+)\s+(\S+)", "w1 w2", no_expand!("$$1"), "$$1");
  use_!(Captures);
  replace!(closure_returning_reference, replace, r"(\d+)", "age: 26",
  - | captures: &Captures | &match_text!(captures.get(1).unwrap())[0..1], "age: 2");
  + |captures: &Captures| {
  + match_text!(captures.get(1).unwrap())[0..1].to_owned()
  + }, "age: 2");
  replace!(closure_returning_value, replace, r"\d+", "age: 26",
  |_captures: &Captures| t!("Z").to_owned(), "age: Z");
  