Fix comment describing what the test does

Author: mejrs

tests/ui/on-unimplemented/expected-comma-found-token.rs

@@ -1,13 +1,10 @@
-// Tests that two closures cannot simultaneously have mutable
-// access to the variable, whether that mutable access be used
-// for direct assignment or for taking mutable ref. Issue #6801.
+//! Test for invalid MetaItem syntax in the attribute
 
+#![crate_type = "lib"]
 #![feature(rustc_attrs)]
 
 #[rustc_on_unimplemented(
     message="the message"
     label="the label" //~ ERROR expected `,`, found `label`
 )]
 trait T {}
-
-fn main() {  }

tests/ui/on-unimplemented/expected-comma-found-token.stderr

@@ -1,5 +1,5 @@
 error: expected `,`, found `label`
-  --> $DIR/expected-comma-found-token.rs:9:5
+  --> $DIR/expected-comma-found-token.rs:8:5
    |
 LL |     message="the message"
    |                          - expected `,`