Use writeln!(fmt, "word") instead of write!(fmt, "word\n") (clippy::write_with_newline)

src/libstd/sys_common/backtrace.rs

@@ -70,7 +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")?;
+    writeln!(fmt, "stack backtrace:")?;
     let mut bt_fmt = BacktraceFmt::new(fmt, print_fmt, &mut print_path);
     bt_fmt.add_context()?;
     let mut idx = 0;

src/libtest/formatters/mod.rs

@@ -36,5 +36,5 @@ pub(crate) fn write_stderr_delimiter(test_output: &mut Vec<u8>, test_name: &Test
         Some(_) => test_output.push(b'\n'),
         None => (),
     }
-    write!(test_output, "---- {} stderr ----\n", test_name).unwrap();
+    writeln!(test_output, "---- {} stderr ----", test_name).unwrap();
 }