Add test for let outside function recovery

Author: obeis

tests/ui/parser/let-outside-function.rs

@@ -0,0 +1,5 @@
+let a = 1;
+//~^ ERROR `let` is invalid outside of a function
+
+fn main() {
+}

tests/ui/parser/let-outside-function.stderr

@@ -0,0 +1,17 @@
+error: `let` is invalid outside of a function
+  --> $DIR/let-outside-function.rs:1:1
+   |
+LL | let a = 1;
+   | ^^^
+   |
+help: consider using `static` instead of `let`
+   |
+LL | static a = 1;
+   | ~~~~~~
+help: consider using `const` instead of `let`
+   |
+LL | const a = 1;
+   | ~~~~~
+
+error: aborting due to previous error
+

tests/ui/parser/suggest-const-for-global-var.rs

@@ -1,5 +1,5 @@
 let X: i32 = 12;
-//~^ ERROR expected item, found keyword `let`
+//~^ ERROR `let` is invalid outside of a function
 
 fn main() {
     println!("{}", X);

tests/ui/parser/suggest-const-for-global-var.stderr

@@ -1,8 +1,17 @@
-error: expected item, found keyword `let`
+error: `let` is invalid outside of a function
   --> $DIR/suggest-const-for-global-var.rs:1:1
    |
 LL | let X: i32 = 12;
-   | ^^^ consider using `const` or `static` instead of `let` for global variables
+   | ^^^
+   |
+help: consider using `static` instead of `let`
+   |
+LL | static X: i32 = 12;
+   | ~~~~~~
+help: consider using `const` instead of `let`
+   |
+LL | const X: i32 = 12;
+   | ~~~~~
 
 error: aborting due to previous error