fix: `flutter` command gets locked when trying to run sharded tests parallely

[RishabJaiswal]

Description
When running multiple flutter test commands parallely via flutterTest() method for sharded tests, the cli throws the below error

Waiting for another flutter command to release the startup lock...

If the same flutter test command is ran for sharded tests without using flutterTest() method but via Process.start, the flutter command runs the sharded tests successfully.

Steps To Reproduce

1. Call the below methods simultaneously

flutterTest(
          arguments:
          "--total-shards 2 --shard-index 0 '${testDirectory.path}/example_a[1]_test.dart'"
              .split(' '),
          workingDirectory: tempDirectory.path,
        );

flutterTest(
          arguments:
          "--total-shards 2 --shard-index 1 '${testDirectory.path}/example_a[2]_test.dart'"
              .split(' '),
          workingDirectory: tempDirectory.path,
        );

Expected Behavior
When executing the flutter test command via Process.start for the shared tests, it runs without failure.
flutterTest() method also uses Process.start internally & it should run without locking flutter command on the sharded tests.

Process.start(
    'flutter',
    "--total-shards 2 --shard-index 1 '${testDirectory.path}/example_a[1]_test.dart'"
              .split(' '),
  );
Process.start(
    'flutter',
    "--total-shards 2 --shard-index 2 '${testDirectory.path}/example_a[2]_test.dart'"
              .split(' '),
  );

Screenshots

Additional Context
Please find the test case to reproduce the issue here:

test('emits correctly for sharded tests (e2e)', () async {
      final tempDirectory = Directory.systemTemp.createTempSync();
      File('${tempDirectory.path}/pubspec.yaml').writeAsStringSync(
        '''
name: example
version: 0.1.0+1

environment:
  sdk: ">=2.12.0 <3.0.0"

dev_dependencies:
  test: any
''',
      );
      final testDirectory = Directory('${tempDirectory.path}/test')
        ..createSync();
      File('${testDirectory.path}/example_a[1]_test.dart').writeAsStringSync(
        '''
import 'package:test/test.dart';

void main() {
  test('example', () {
    expect(true, isTrue);
  });
}
''',
      );

      File('${testDirectory.path}/example_a[2]_test.dart').writeAsStringSync(
        '''
import 'package:test/test.dart';

void main() {
  test('example', () {
    expect(true, isTrue);
  });
}
''',
      );
      expect(
        flutterTest(
          arguments:
              "--total-shards 2 --shard-index 0 '${testDirectory.path}/example_a[1]_test.dart'"
                  .split(' '),
          workingDirectory: tempDirectory.path,
        ).where((e) => e is DoneTestEvent).first,
        completes,
      );

      expect(
        flutterTest(
          arguments:
          "--total-shards 2 --shard-index 1 '${testDirectory.path}/example_a[2]_test.dart'"
              .split(' '),
          workingDirectory: tempDirectory.path,
        ).where((e) => e is DoneTestEvent).first,
        completes,
      );
    });