test: Fail early if the local yarn build is missing (#6351)

Previously if the tests were run without first having run `yarn build`,
these three test suites would fail with over 1000 lines of unclear
console output.

Now, later tests are stopped from running to reduce verbosity, and a
more contributor-friendly error message displayed instead.

We unfortunately can't handle this case in `beforeAll()`, since even
if it throws, later tests in the same file are still run:
jestjs/jest#2713

__tests__/index.js

@@ -1,5 +1,7 @@
 /* @flow */
 
+import {existsSync} from 'fs';
+
 import NoopReporter from '../src/reporters/base-reporter.js';
 import makeTemp from './_temp';
 import * as fs from '../src/util/fs.js';
@@ -16,6 +18,10 @@ ver = ver.split('-')[0];
 
 jasmine.DEFAULT_TIMEOUT_INTERVAL = 60000;
 
+if (!existsSync(path.resolve(__dirname, '../lib'))) {
+  throw new Error('These tests require `yarn build` to have been run first.');
+}
+
 async function execCommand(
   cmd: string,
   args: Array<string>,

__tests__/integration.js

@@ -2,6 +2,7 @@
 /* eslint max-len: 0 */
 
 import http from 'http';
+import {existsSync} from 'fs';
 
 import invariant from 'invariant';
 import execa from 'execa';
@@ -16,6 +17,10 @@ jasmine.DEFAULT_TIMEOUT_INTERVAL = 120000;
 
 const path = require('path');
 
+if (!existsSync(path.resolve(__dirname, '../lib'))) {
+  throw new Error('These tests require `yarn build` to have been run first.');
+}
+
 function addTest(pattern, {strictPeers} = {strictPeers: false}, yarnArgs: Array<string> = []) {
   test.concurrent(`yarn add ${pattern}`, async () => {
     const cwd = await makeTemp();

__tests__/lifecycle-scripts.js

@@ -1,5 +1,7 @@
 /* @flow */
 
+import {existsSync} from 'fs';
+
 import NoopReporter from '../src/reporters/base-reporter.js';
 import makeTemp from './_temp';
 import * as fs from '../src/util/fs.js';
@@ -12,6 +14,10 @@ const yarnBin = path.join(__dirname, '../bin/yarn.js');
 
 jasmine.DEFAULT_TIMEOUT_INTERVAL = 60000;
 
+if (!existsSync(path.resolve(__dirname, '../lib'))) {
+  throw new Error('These tests require `yarn build` to have been run first.');
+}
+
 async function execCommand(cmd: string, packageName: string, env = process.env): Promise<string> {
   const srcPackageDir = path.join(fixturesLoc, packageName);
   const packageDir = await makeTemp(packageName);