fix lazy css bug, add tests

test/apps/css/src/server.js

@@ -0,0 +1,8 @@
+import polka from 'polka';
+import * as sapper from '../__sapper__/server.js';
+
+const { PORT } = process.env;
+
+polka()
+	.use(sapper.middleware())
+	.listen(PORT);