Add test for attribute parser but has unexpected error

Author: tiif

tests/ui/internal/unstable-feature.rs

@@ -0,0 +1,7 @@
+#![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
+fn main() {
+
+}

tests/ui/internal/unstable-feature.stderr

@@ -0,0 +1,14 @@
+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
+