add package.mask entry to mock repo

tests/profile/mockrepo/profiles/features/emacs/package.mask

@@ -0,0 +1 @@
+app-editors/vim

tests/profile/read_mock_profile.rs

@@ -27,6 +27,15 @@ fn main() {
         vec!["app-editors/emacs"]
     );
 
+    assert_eq!(
+        profile
+            .package_mask()
+            .iter()
+            .map(Atom::to_string)
+            .collect::<Vec<_>>(),
+        vec!["app-editors/vim"]
+    );
+
     let emacs_package_use = profile
         .package_use()
         .iter()