[engineering] add testSplit option to unit-test runner (microsoft#253049)

helpful to run tests in parallel

Author: tmm1

test/unit/electron/index.js

@@ -30,6 +30,7 @@ const minimist = require('minimist');
  * grep: string;
  * run: string;
  * runGlob: string;
+ * testSplit: string;
  * dev: boolean;
  * reporter: string;
  * 'reporter-options': string;
@@ -46,7 +47,7 @@ const minimist = require('minimist');
  * }}
  */
 const args = minimist(process.argv.slice(2), {
-	string: ['grep', 'run', 'runGlob', 'reporter', 'reporter-options', 'waitServer', 'timeout', 'crash-reporter-directory', 'tfs', 'coveragePath', 'coverageFormats'],
+	string: ['grep', 'run', 'runGlob', 'reporter', 'reporter-options', 'waitServer', 'timeout', 'crash-reporter-directory', 'tfs', 'coveragePath', 'coverageFormats', 'testSplit'],
 	boolean: ['build', 'coverage', 'help', 'dev', 'per-test-coverage'],
 	alias: {
 		'grep': ['g', 'f'],
@@ -67,6 +68,7 @@ Options:
 --grep, -g, -f <pattern>      only run tests matching <pattern>
 --run <file>                  only run tests from <file>
 --runGlob, --glob, --runGrep <file_pattern> only run tests matching <file_pattern>
+--testSplit <i>/<n>           split tests into <n> parts and run the <i>th part
 --build                       run with build output (out-build)
 --coverage                    generate coverage report
 --per-test-coverage           generate a per-test V8 coverage report, only valid with the full-json-stream reporter

test/unit/electron/renderer.js

@@ -171,7 +171,14 @@ async function loadTestModules(opts) {
 
 	const pattern = opts.runGlob || _tests_glob;
 	const files = await globAsync(pattern, { cwd: loadFn._out });
-	const modules = files.map(file => file.replace(/\.js$/, ''));
+	let modules = files.map(file => file.replace(/\.js$/, ''));
+	if (opts.testSplit) {
+		const [i, n] = opts.testSplit.split('/').map(Number);
+		const chunkSize = Math.floor(modules.length / n);
+		const start = (i - 1) * chunkSize;
+		const end = i === n ? modules.length : i * chunkSize;
+		modules = modules.slice(start, end);
+	}
 	return loadModules(modules);
 }