Fix failing backtrace ui tests

Author: yaahc

src/libstd/sys_common/backtrace.rs

@@ -70,6 +70,7 @@ unsafe fn _print_fmt(fmt: &mut fmt::Formatter<'_>, print_fmt: PrintFmt) -> fmt::
     let mut print_path = move |fmt: &mut fmt::Formatter<'_>, bows: BytesOrWideString<'_>| {
         output_filename(fmt, bows, print_fmt, cwd.as_ref())
     };
+    write!(fmt, "stack backtrace:\n")?;
     let mut bt_fmt = BacktraceFmt::new(fmt, print_fmt, &mut print_path);
     bt_fmt.add_context()?;
     let mut idx = 0;

src/test/ui/std-backtrace.rs

@@ -16,9 +16,9 @@ use std::str;
 fn main() {
     let args: Vec<String> = env::args().collect();
     if args.len() >= 2 && args[1] == "force" {
-        println!("{}", std::backtrace::Backtrace::force_capture());
+        println!("stack backtrace:\n{}", std::backtrace::Backtrace::force_capture());
     } else if args.len() >= 2 {
-        println!("{}", std::backtrace::Backtrace::capture());
+        println!("stack backtrace:\n{}", std::backtrace::Backtrace::capture());
     } else {
         runtest(&args[0]);
         println!("test ok");