Add CSS variables test

test/cssvars.js

@@ -0,0 +1,13 @@
+const Metalsmith = require('metalsmith');
+const filter = require('metalsmith-filter');
+
+const regex_sass = '**/*.sass';
+const other_plugin = require('./plugins/check-css-variables-exist');
+
+Metalsmith(__dirname)
+  .source('../sass')
+  .use(filter(regex_sass))
+  .use(other_plugin())
+  .build(function(err) {
+    if (err) throw err;
+  });