Add failing test for rust-lang#3778 write_with_newline

Literal `\n` characters (not a newline) in a `r"raw"` string should not
fail the lint.

Author: uniphil

tests/ui/write_with_newline.rs

@@ -31,4 +31,7 @@ fn main() {
     write!(&mut v, "\\n"); // #3514
     write!(&mut v, "\\\n");
     write!(&mut v, "\\\\n");
+
+    // Raw strings
+    write!(&mut v, r"\n"); // #3778
 }