Add test concurrency flag

doc/api/test.md

@@ -331,9 +331,14 @@ changes:
   does not have a name.
 * `options` {Object} Configuration options for the test. The following
   properties are supported:
-  * `concurrency` {number} The number of tests that can be run at the same time.
+  * `concurrency` {number|boolean} If a number is provided,
+    then that many tests would run in parallel.
+    If truthy, it would run (number of cpu cores - 1)
+    tests in parallel.
+    For subtests, it will be `Infinity` tests in parallel.
+    If falsy, it would only run one test at a time.
     If unspecified, subtests inherit this value from their parent.
-    **Default:** `1`.
+    **Default:** `false`.
   * `only` {boolean} If truthy, and the test context is configured to run
     `only` tests, then this test will be run. Otherwise, the test is skipped.
     **Default:** `false`.

lib/internal/test_runner/test.js

@@ -4,6 +4,7 @@ const {
   ArrayPrototypeShift,
   ArrayPrototypeUnshift,
   FunctionPrototype,
+  MathMax,
   Number,
   PromisePrototypeThen,
   PromiseResolve,
@@ -47,8 +48,7 @@ const noop = FunctionPrototype;
 const isTestRunner = getOptionValue('--test');
 const testOnlyFlag = !isTestRunner && getOptionValue('--test-only');
 // TODO(cjihrig): Use uv_available_parallelism() once it lands.
-const rootConcurrency = isTestRunner ? cpus().length : 1;
-
+const rootConcurrency = isTestRunner ? MathMax(cpus().length - 1, 1) : 1;
 const kShouldAbort = Symbol('kShouldAbort');
 
 
@@ -146,6 +146,12 @@ class Test extends AsyncResource {
 
     if (isUint32(concurrency) && concurrency !== 0) {
       this.concurrency = concurrency;
+    } else if (typeof concurrency === 'boolean') {
+      if (concurrency) {
+        this.concurrency = isTestRunner ? MathMax(cpus().length - 1, 1) : Infinity;
+      } else {
+        this.concurrency = 1;
+      }
     }
 
     if (isUint32(timeout)) {

test/parallel/test-runner-concurrency.js

@@ -0,0 +1,29 @@
+'use strict';
+require('../common');
+const { describe, it } = require('node:test');
+const assert = require('assert');
+
+describe('Concurrency option (boolean) = true ', { concurrency: true }, () => {
+  let isFirstTestOver = false;
+  it('should start the first test', () => new Promise((resolve) => {
+    setImmediate(() => { isFirstTestOver = true; resolve(); });
+  }));
+  it('should start before the previous test ends', () => {
+    // Should work even on single core CPUs
+    assert.strictEqual(isFirstTestOver, false);
+  });
+});
+
+describe(
+  'Concurrency option (boolean) = false ',
+  { concurrency: false },
+  () => {
+    let isFirstTestOver = false;
+    it('should start the first test', () => new Promise((resolve) => {
+      setImmediate(() => { isFirstTestOver = true; resolve(); });
+    }));
+    it('should start after the previous test ends', () => {
+      assert.strictEqual(isFirstTestOver, true);
+    });
+  }
+);