Add test for println! typo

Author: jhpratt

tests/ui/did_you_mean/println-typo.rs

@@ -0,0 +1,6 @@
+// https://internals.rust-lang.org/t/18227
+
+fn main() {
+    prinltn!(); //~ ERROR cannot find macro `prinltn` in this scope
+    //^ a macro with a similar name exists: `println`
+}

tests/ui/did_you_mean/println-typo.stderr

@@ -0,0 +1,11 @@
+error: cannot find macro `prinltn` in this scope
+  --> $DIR/println-typo.rs:4:5
+   |
+LL |     prinltn!();
+   |     ^^^^^^^ help: a macro with a similar name exists: `println`
+  --> $SRC_DIR/std/src/macros.rs:LL:COL
+   |
+   = note: similarly named macro `println` defined here
+
+error: aborting due to previous error
+