test: enable parallel execution of Mocha tests

.mocharc.js

@@ -6,8 +6,13 @@
 const {mergeMochaConfigs} = require('./packages/build');
 const defaultConfig = require('./packages/build/config/.mocharc.json');
 
+const MONOREPO_CONFIG = {
+  parallel: true,
+};
+
 module.exports = mergeMochaConfigs(
   defaultConfig,
+  MONOREPO_CONFIG,
   // Apply Mocha config from packages that require custom Mocha setup
   require('./packages/cli/.mocharc.js'),
 );

docs/site/DEVELOPING.md

@@ -142,7 +142,7 @@ npm test
 
 It does all you need:
 
-- Compile TypeScript
+- Compile TypeScript (full rebuild)
 - Run all tests
 - Check code formatting using [Prettier](https://prettier.io/)
 - Lint the code using [ESLint](https://typescript-eslint.io/)
@@ -161,6 +161,40 @@ Or use a simpler form on Mac and Linux.
 CI=1 npm test
 ```
 
+Running the full test suite after each small change is not very effective. You
+can use the following commands to run a subset of checks:
+
+- `npm run build` for a fast incremental build
+- `npm run mocha` to (re)run the test suite
+
+We are running tests in parallel, use the Mocha option `-j` (`--jobs`) to
+control the number of worker processes or disable parallel execution entirely by
+using a single job only:
+
+```
+$ npm run mocha -- -j 1
+```
+
+When working in a single package, it's possible to limit the compilation to this
+package & its dependencies and then run only package tests.
+
+For example, run the following Unix command in `loopback-next` root directory to
+build and test changes made in `@loopback/rest` only:
+
+```
+$ (cd packages/rest && npm t)
+```
+
+On Windows, you have to change the working directory once and then you can
+repeatedly run `npm t` inside the package.
+
+```bat
+rem Run this only once
+cd packages/rest
+rem Run this to build & test your changes
+npm t
+```
+
 ## Coding rules
 
 - All features and bug fixes must be covered by one or more automated tests.

package.json

@@ -70,7 +70,7 @@
     "build:site": "./bin/build-docs-site.sh",
     "docs:prepare": "./docs/bin/build-preview-site.sh",
     "docs:start": "cd docs/_preview && bundle exec jekyll serve --no-w --i",
-    "mocha": "node packages/build/bin/run-mocha --lang en_US.UTF-8 --timeout 5000 \"packages/*/dist/__tests__/**/*.js\" \"extensions/*/dist/__tests__/**/*.js\" \"examples/*/dist/__tests__/**/*.js\" \"packages/cli/test/**/*.js\" \"packages/build/test/*/*.js\"",
+    "mocha": "node packages/build/bin/run-mocha --lang en_US.UTF-8 \"packages/*/dist/__tests__/**/*.js\" \"extensions/*/dist/__tests__/**/*.js\" \"examples/*/dist/__tests__/**/*.js\" \"packages/cli/test/**/*.js\" \"packages/build/test/*/*.js\"",
     "posttest": "npm run lint"
   },
   "config": {

packages/cli/test/integration/cli/cli.integration.js

@@ -49,7 +49,10 @@ describe('cli', () => {
     expect(entries).to.not.containEql('Available commands:');
   });
 
-  it('saves command metadata to .yo-rc.json', () => {
+  it('saves command metadata to .yo-rc.json', function () {
+    // This test can be slow under parallel mode
+    // eslint-disable-next-line @typescript-eslint/no-invalid-this
+    this.timeout(15000);
     const entries = [];
     main({meta: true}, getLog(entries));
     const logs = entries.join('');