Add test for raw string

Author: estebank

src/test/ui/ifmt-bad-arg.rs

@@ -64,4 +64,11 @@ fn main() {
     format!("foo }"); //~ ERROR: unmatched `}` found
 
     format!("foo %s baz", "bar"); //~ ERROR: argument never used
+
+    format!(r##"
+
+        {foo}
+
+    "##);
+    //~^^^ ERROR: there is no argument named `foo`
 }

src/test/ui/ifmt-bad-arg.stderr

@@ -183,5 +183,11 @@ LL |     format!("foo %s baz", "bar"); //~ ERROR: argument never used
    = help: `%s` should be written as `{}`
    = note: printf formatting not supported; see the documentation for `std::fmt`
 
-error: aborting due to 26 previous errors
+error: there is no argument named `foo`
+  --> $DIR/ifmt-bad-arg.rs:70:9
+   |
+LL |         {foo}
+   |         ^^^^^
+
+error: aborting due to 27 previous errors