doc,test: fix concurrency option of test()

doc/api/test.md

@@ -812,9 +812,8 @@ changes:
   properties are supported:
   * `concurrency` {number|boolean} If a number is provided,
     then that many tests would run in parallel within the application thread.
-    If `true`, it would run `os.availableParallelism() - 1` tests in parallel.
-    For subtests, it will be `Infinity` tests in parallel.
-    If `false`, it would only run one test at a time.
+    If `true`, all scheduled asynchronous tests run concurrently within the
+    thread. If `false`, only one test runs at a time.
     If unspecified, subtests inherit this value from their parent.
     **Default:** `false`.
   * `only` {boolean} If truthy, and the test context is configured to run

test/parallel/test-runner-concurrency.js

@@ -7,6 +7,7 @@ const assert = require('node:assert');
 const path = require('node:path');
 const fs = require('node:fs/promises');
 const os = require('node:os');
+const timers = require('node:timers/promises');
 
 tmpdir.refresh();
 
@@ -35,6 +36,22 @@ describe(
   }
 );
 
+// Despite the docs saying so at some point, setting concurrency to true should
+// not limit concurrency to the number of available CPU cores.
+describe('concurrency: true implies Infinity', { concurrency: true }, () => {
+  // The factor 5 is intentionally chosen to be higher than the default libuv
+  // thread pool size.
+  const nTests = 5 * os.availableParallelism();
+  let nStarted = 0;
+  for (let i = 0; i < nTests; i++) {
+    it(`should run test ${i} concurrently`, async () => {
+      assert.strictEqual(nStarted++, i);
+      await timers.setImmediate();
+      assert.strictEqual(nStarted, nTests);
+    });
+  }
+});
+
 {
   // Make sure tests run in order when root concurrency is 1 (default)
   const tree = [];