Fix the test by using correct feature gate name

Author: tiif

compiler/rustc_feature/src/builtin_attrs.rs

@@ -656,8 +656,8 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         "allow_internal_unstable side-steps feature gating and stability checks",
     ),
     gated!(
-        impl_stability, Normal, template!(Word, List: "feat1, feat2, ..."),
-        DuplicatesOk, EncodeCrossCrate::Yes,
+        allow_unstable_feature, Normal, template!(Word, List: "feat1, feat2, ..."),
+        DuplicatesOk, EncodeCrossCrate::No, impl_stability,
         "allow unstable impl",
     ),
     gated!(

tests/ui/internal/unstable-feature.rs

@@ -1,7 +1,6 @@
 #![feature(impl_stability)] 
 
-#[allow_unstable_feature]  //~ ERROR cannot find attribute `allow_unstable_feature` in this scope
-//~^ ERROR `allow_unstable_feature` expects a list of feature names
+#[allow_unstable_feature]  //~ ERROR `allow_unstable_feature` expects a list of feature names
 fn main() {
 
 }

tests/ui/internal/unstable-feature.stderr

@@ -1,14 +1,8 @@
-error: cannot find attribute `allow_unstable_feature` in this scope
-  --> $DIR/unstable-feature.rs:3:3
-   |
-LL | #[allow_unstable_feature]
-   |   ^^^^^^^^^^^^^^^^^^^^^^
-
 error: `allow_unstable_feature` expects a list of feature names
   --> $DIR/unstable-feature.rs:3:1
    |
 LL | #[allow_unstable_feature]
    | ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error