test error message for undefined mixin and variable

tests/mixins.rs

@@ -229,3 +229,7 @@ error!(
     "a {\n    @if true {\n        @content;\n    }\n}\n",
     "Error: @content is only allowed within mixin declarations."
 );
+error!(
+    undefined_mixin,
+    "a {@include foo;}", "Error: Undefined mixin."
+);

tests/variables.rs

@@ -135,3 +135,7 @@ test!(
 );
 error!(ends_with_bang, "$a: red !;", "Error: Expected identifier.");
 error!(unknown_flag, "$a: red !foo;", "Error: Invalid flag name.");
+error!(
+    undefined_variable,
+    "a {color: $a;}", "Error: Undefined variable."
+);