Update tests for new format! error message

src/test/ui/cross-crate-macro-backtrace/main.stderr

@@ -1,4 +1,4 @@
-error: invalid reference to argument `0` (no arguments given)
+error: 1 positional argument in format string, but no arguments were given
   --> $DIR/main.rs:16:5
    |
 16 |     myprintln!("{}"); //~ ERROR in this macro

src/test/ui/macros/macro-backtrace-println.stderr

@@ -1,4 +1,4 @@
-error: invalid reference to argument `0` (no arguments given)
+error: 1 positional argument in format string, but no arguments were given
   --> $DIR/macro-backtrace-println.rs:24:30
    |
 24 |     ($fmt:expr) => (myprint!(concat!($fmt, "/n")));