Add test for rustX codeblock attribute

Author: GuillaumeGomez

Diff for: tests/rustdoc-ui/lints/check-attr.rs

@@ -39,3 +39,11 @@ pub fn foobar() {}
 /// boo
 /// ```
 pub fn b() {}
+
+/// b
+//~^ ERROR
+///
+/// ```rust2018
+/// boo
+/// ```
+pub fn c() {}

Diff for: tests/rustdoc-ui/lints/check-attr.stderr

@@ -171,5 +171,16 @@ LL | | /// ```
    |
    = help: the code block will either not be tested if not marked as a rust one or the code will be wrapped inside a main function
 
-error: aborting due to 12 previous errors
+error: unknown attribute `rust2018`. Did you mean `edition2018`?
+  --> $DIR/check-attr.rs:43:1
+   |
+LL | / /// b
+LL | |
+LL | | ///
+LL | | /// ```rust2018
+LL | | /// boo
+LL | | /// ```
+   | |_______^
+
+error: aborting due to 13 previous errors