Fix that `flutter test` does not understand `concurrency` (#125942)

Close https://github.com/flutter/flutter/issues/125940

I will add tests if this PR looks roughly OK :)

The fix mainly mimics https://github.com/flutter/flutter/pull/115160 - just remove the default argument.

p.s. I ran into this bug when wanting to set concurrency in my dart_test.yaml for one set of my tests which I need to be executed without parallalization.

packages/flutter_tools/lib/src/commands/test.dart

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'dart:math' as math;
-
 import 'package:meta/meta.dart';
 
 import '../asset.dart';
@@ -146,7 +144,6 @@ class TestCommand extends FlutterCommand with DeviceBasedDevelopmentArtifacts {
       )
       ..addOption('concurrency',
         abbr: 'j',
-        defaultsTo: math.max<int>(1, globals.platform.numberOfProcessors - 2).toString(),
         help: 'The number of concurrent test processes to run. This will be ignored '
               'when running integration tests.',
         valueHelp: 'jobs',
@@ -353,8 +350,9 @@ class TestCommand extends FlutterCommand with DeviceBasedDevelopmentArtifacts {
       );
     }
 
-    int? jobs = int.tryParse(stringArg('concurrency')!);
-    if (jobs == null || jobs <= 0 || !jobs.isFinite) {
+    final String? concurrencyString = stringArg('concurrency');
+    int? jobs = concurrencyString == null ? null : int.tryParse(concurrencyString);
+    if (jobs != null && (jobs <= 0 || !jobs.isFinite)) {
       throwToolExit(
         'Could not parse -j/--concurrency argument. It must be an integer greater than zero.'
       );

packages/flutter_tools/lib/src/test/runner.dart

@@ -109,6 +109,7 @@ class _FlutterTestRunnerImpl implements FlutterTestRunner {
         '--file-reporter=$fileReporter',
       if (timeout != null)
         ...<String>['--timeout', timeout],
+      if (concurrency != null)
         '--concurrency=$concurrency',
       for (final String name in names)
         ...<String>['--name', name],

packages/flutter_tools/test/commands.shard/hermetic/test_test.dart

@@ -158,7 +158,7 @@ dev_dependencies:
   });
 
   testUsingContext(
-      'Confirmation that the reporter and timeout args are not set by default',
+      'Confirmation that the reporter, timeout, and concurrency args are not set by default',
       () async {
     final FakePackageTest fakePackageTest = FakePackageTest();
 
@@ -175,6 +175,7 @@ dev_dependencies:
     expect(fakePackageTest.lastArgs, isNot(contains('compact')));
     expect(fakePackageTest.lastArgs, isNot(contains('--timeout')));
     expect(fakePackageTest.lastArgs, isNot(contains('30s')));
+    expect(fakePackageTest.lastArgs, isNot(contains('--concurrency')));
   }, overrides: <Type, Generator>{
     FileSystem: () => fs,
     ProcessManager: () => FakeProcessManager.any(),