Use template-lint test harness

Author: dcyriller

test/unit/rules/lint-prettier-test.js

@@ -1,8 +1,14 @@
-const generateRuleTests = require("../../helpers/rule-test-harness");
+const generateRuleTests = require("ember-template-lint/lib/helpers/rule-test-harness");
+const plugin = require("../../../ember-template-lint-plugin-prettier");
 
 generateRuleTests({
   name: "prettier",
 
+  groupMethodBefore: beforeEach,
+  groupingMethod: describe,
+  testMethod: it,
+  plugins: [plugin],
+
   good: [
     ``,
     `{{! template-lint-disable prettier}}