test: check replaced iff translatable comment

All translatable comments should be replaced, but nothing else.

curriculum/test/utils/extract-html-comments.js

@@ -0,0 +1,12 @@
+var rehype = require('rehype');
+var vfile = require('vfile');
+
+var getComments = require('./plugins/get-html-comments');
+
+const processor = rehype().use(getComments);
+
+function extractComments(html) {
+  return processor.processSync(vfile(html)).data;
+}
+
+module.exports = extractComments;