Add test for format_args!("{}", 0) in const.

tests/ui/consts/const-eval/format.rs

@@ -9,4 +9,9 @@ const fn print() {
     //~| ERROR cannot call non-const function `_print` in constant functions
 }
 
+const fn format_args() {
+    format_args!("{}", 0);
+    //~^ ERROR cannot call non-const formatting macro in constant functions
+}
+
 fn main() {}

tests/ui/consts/const-eval/format.stderr

@@ -24,6 +24,14 @@ LL |     println!("{:?}", 0);
    = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
    = note: this error originates in the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
 
-error: aborting due to 3 previous errors
+error[E0015]: cannot call non-const formatting macro in constant functions
+  --> $DIR/format.rs:13:5
+   |
+LL |     format_args!("{}", 0);
+   |     ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0015`.