Add some fixture utils and assert that the .prettierrc.js is generate…

…d correctly for all tests, ensuring that the fixture utils work

tests/.eslintignore

@@ -0,0 +1 @@
+fixtures/

tests/.prettierignore

@@ -0,0 +1 @@
+fixtures/

tests/assertions.ts

@@ -1,8 +1,9 @@
 import fse from 'fs-extra';
+import fs from 'node:fs/promises';
 import path from 'node:path';
 import { expect } from 'vitest';
 
-import { packageJsonAt } from './utils.js';
+import { fixture, packageJsonAt } from './utils.js';
 
 interface AssertGeneratedOptions {
   projectRoot: string;
@@ -50,4 +51,45 @@ export async function assertGeneratedCorrectly({
 
     expect(await fse.pathExists(pathToFile), `${pathToFile} exists`).toBe(true);
   }
+
+  await matchesFixture('.prettierrc.js', { cwd: projectRoot });
+}
+
+export async function matchesFixture(
+  /**
+   * Project-relative file to test against
+   */
+  testFilePath: string,
+  options?: {
+    /**
+     * Which fixture set to use
+     */
+    scenario?: string;
+    /**
+     * By default, the file used will be the same as the testFilePath, but
+     * in the fixtures directory under the (maybe) specified scenario.
+     * this can be overridden, if needed.
+     * (like if you're testFilePath is deep with in an existing monorepo, and wouldn't
+     *   inherently match our default-project structure used in the fixtures)
+     */
+    file?: string;
+
+    /**
+     * The working directory to use for the relative paths. Defaults to process.cwd() (node default)
+     */
+    cwd?: string;
+  }
+) {
+  let scenario = options?.scenario ?? 'default';
+  let fixtureFile = options?.file ?? testFilePath;
+
+  if (options?.cwd) {
+    testFilePath = path.join(options.cwd, testFilePath);
+    fixtureFile = path.join(options.cwd, fixtureFile);
+  }
+
+  let sourceContents = await fs.readFile(testFilePath);
+  let fixtureContents = await fixture(fixtureFile, { scenario });
+
+  expect(sourceContents).to.equal(fixtureContents, `${testFilePath} matches ${fixtureFile}`);
 }

tests/fixtures/default/.prettierrc.js

@@ -0,0 +1,5 @@
+'use strict';
+
+module.exports = {
+  singleQuote: true,
+};

tests/utils.ts

@@ -8,9 +8,29 @@ import { fileURLToPath } from 'node:url';
 const __dirname = path.dirname(fileURLToPath(import.meta.url));
 
 const blueprintPath = path.join(__dirname, '..');
+const fixturesPath = path.join(__dirname, 'fixtures');
 
 export const SUPPORTED_PACKAGE_MANAGERS = ['npm', 'yarn', 'pnpm'] as const;
 
+export async function fixture(
+  /**
+   * Which file within in the fixture-set / scenario to read
+   */
+  file: string,
+  options?: {
+    /**
+     * Which fixture set to use
+     */
+    scenario?: string;
+  }
+) {
+  let scenario = options?.scenario ?? 'default';
+  let fixtureFilePath = path.join(fixturesPath, scenario, file);
+  let contents = await fs.readFile(fixtureFilePath);
+
+  return contents.toString();
+}
+
 export async function createTmp() {
   let prefix = 'v2-addon-blueprint--';
   let prefixPath = path.join(os.tmpdir(), prefix);