build: Allow using jest as a test framework (#1955)

If a package's `package.json` file contains a `jest` entry (used for
configuring the behavior of `jest`), assert the package uses `jest`
instead of `nodeunit` as it's test harness (and fail if there are any
`nodeunit` tests - based on file name).

This is going to make it easier to migrate away from `nodeunit`.

tools/cdk-build-tools/bin/cdk-test.ts

@@ -2,14 +2,20 @@ import fs = require('fs');
 import util = require('util');
 import yargs = require('yargs');
 import { shell } from '../lib/os';
-import { cdkBuildOptions, configFilePath, hasIntegTests, hasOnlyAutogeneratedTests, unitTestFiles } from '../lib/package-info';
+import { cdkBuildOptions, configFilePath, currentPackageJson, hasIntegTests, hasOnlyAutogeneratedTests, unitTestFiles } from '../lib/package-info';
 import { Timers } from '../lib/timer';
 
 async function main() {
   const args = yargs
     .env('CDK_TEST')
     .usage('Usage: cdk-test')
     .option('force', { type: 'boolean', alias: 'f', desc: 'Force a rebuild' })
+    .option('jest', {
+      type: 'string',
+      desc: 'Specify a different jest executable',
+      default: require.resolve('jest/bin/jest'),
+      defaultDescription: 'jest provided by node dependencies'
+    })
     .option('nyc', {
       type: 'string',
       desc: 'Specify a different nyc executable',
@@ -31,7 +37,14 @@ async function main() {
   }
 
   const testFiles = await unitTestFiles();
-  if (testFiles.length > 0) {
+  const useJest = 'jest' in currentPackageJson();
+
+  if (useJest) {
+    if (testFiles.length > 0) {
+      throw new Error(`Jest is enabled, but ${testFiles.length} nodeunit tests were found!`);
+    }
+    await shell([args.jest, '--testEnvironment=node', '--coverage'], timers);
+  } else if (testFiles.length > 0) {
     const testCommand: string[] = [];
 
     // We always run the tests, but include an 'nyc' run (for coverage)