test_runner: introduces test_concurrency flag

adds the test_concurrency flag,
if set to true, runs all the tests in parallel

Fixes: nodejs#43837

Author: 98lenvi

doc/api/cli.md

@@ -1108,6 +1108,14 @@ Starts the Node.js command line test runner. This flag cannot be combined with
 `--check`, `--eval`, `--interactive`, or the inspector. See the documentation
 on [running tests from the command line][] for more details.
 
+### `--test-concurrency`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+Configures the test runner to run sub tests in parallel.
+
 ### `--test-only`
 
 <!-- YAML

lib/internal/test_runner/test.js

@@ -1,10 +1,12 @@
 'use strict';
 const {
   ArrayPrototypePush,
+  ArrayPrototypeReduce,
   ArrayPrototypeShift,
   ArrayPrototypeUnshift,
   FunctionPrototype,
   Number,
+  PromiseResolve,
   ReflectApply,
   SafeMap,
   PromiseRace,
@@ -41,8 +43,8 @@ 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 ? cpus().length - 1 : 1;
+const subTestConcurrency = getOptionValue('--test-concurrency');
 
 function testTimeout(promise, timeout) {
   if (timeout === kDefaultTimeout) {
@@ -505,7 +507,14 @@ class Suite extends Test {
     this.parent.activeSubtests++;
     this.startTime = hrtime();
     const subtests = this.skipped || this.error ? [] : this.subtests;
-    await SafePromiseAll(subtests, (subtests) => subtests.start());
+    if (subTestConcurrency) {
+      await SafePromiseAll(subtests, (subtests) => subtests.start());
+    } else {
+      await testTimeout(ArrayPrototypeReduce(subtests, async (prev, subtest) => {
+        await prev;
+        await subtest.run();
+      }, PromiseResolve()), this.timeout);
+    }
     this.pass();
     this.postRun();
   }

src/node_options.cc

@@ -525,6 +525,9 @@ EnvironmentOptionsParser::EnvironmentOptionsParser() {
   AddOption("--test",
             "launch test runner on startup",
             &EnvironmentOptions::test_runner);
+  AddOption("--test-concurrency",
+            "tests defined within a file run in parallel",
+            &EnvironmentOptions::test_concurrency);
   AddOption("--test-only",
             "run tests with 'only' option set",
             &EnvironmentOptions::test_only,

src/node_options.h

@@ -150,6 +150,7 @@ class EnvironmentOptions : public Options {
   std::string redirect_warnings;
   std::string diagnostic_dir;
   bool test_runner = false;
+  bool test_concurrency = false;
   bool test_only = false;
   bool test_udp_no_try_send = false;
   bool throw_deprecation = false;