Add test for misspelled feature name

Author: scrabsha

tests/ui/feature-gates/unknown-feature.rs

@@ -1,3 +1,14 @@
-#![feature(unknown_rust_feature)] //~ ERROR unknown feature
+#![feature(
+    unknown_rust_feature,
+    //~^ ERROR unknown feature
+    
+    // Typo for lang feature
+    associated_types_default,
+    //~^ ERROR unknown feature
+
+    // Typo for lib feature
+    core_intrnisics,
+    //~^ ERROR unknown feature
+)]
 
 fn main() {}

tests/ui/feature-gates/unknown-feature.stderr

@@ -1,9 +1,21 @@
 error[E0635]: unknown feature `unknown_rust_feature`
-  --> $DIR/unknown-feature.rs:1:12
+  --> $DIR/unknown-feature.rs:2:5
    |
-LL | #![feature(unknown_rust_feature)]
-   |            ^^^^^^^^^^^^^^^^^^^^
+LL |     unknown_rust_feature,
+   |     ^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 1 previous error
+error[E0635]: unknown feature `associated_types_default`
+  --> $DIR/unknown-feature.rs:6:5
+   |
+LL |     associated_types_default,
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0635]: unknown feature `core_intrnisics`
+  --> $DIR/unknown-feature.rs:10:5
+   |
+LL |     core_intrnisics,
+   |     ^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0635`.