Split tools_tests subshards into separate shards to support sub-sub-sharding (#75033)

dev/bots/test/test_test.dart

@@ -8,6 +8,7 @@ import 'package:file/file.dart' as fs;
 import 'package:file/memory.dart';
 import 'package:mockito/mockito.dart';
 import 'package:path/path.dart' as path;
+import 'package:process/process.dart';
 
 import '../test.dart';
 import 'common.dart';
@@ -80,4 +81,44 @@ void main() {
       expect(actualHash, kSampleHash);
     });
   });
+
+  group('test.dart script', () {
+    const ProcessManager processManager = LocalProcessManager();
+
+    Future<ProcessResult> runScript(
+        [Map<String, String> environment, List<String> otherArgs = const <String>[]]) async {
+      final String dart = path.absolute(
+          path.join('..', '..', 'bin', 'cache', 'dart-sdk', 'bin', 'dart'));
+      final ProcessResult scriptProcess = processManager.runSync(<String>[
+        dart,
+        'test.dart',
+        ...otherArgs,
+      ], environment: environment);
+      return scriptProcess;
+    }
+
+    test('subshards tests correctly', () async {
+      ProcessResult result = await runScript(
+        <String, String>{'SHARD': 'smoke_tests', 'SUBSHARD': '1_3'},
+      );
+      expect(result.exitCode, 0);
+      // There are currently 6 smoke tests. This shard should contain test 1 and 2.
+      expect(result.stdout, contains('Selecting subshard 1 of 3 (range 1-2 of 6)'));
+
+      result = await runScript(
+        <String, String>{'SHARD': 'smoke_tests', 'SUBSHARD': '5_6'},
+      );
+      expect(result.exitCode, 0);
+      // This shard should contain only test 5.
+      expect(result.stdout, contains('Selecting subshard 5 of 6 (range 5-5 of 6)'));
+    });
+
+    test('exits with code 1 when SUBSHARD index greater than total', () async {
+      final ProcessResult result = await runScript(
+        <String, String>{'SHARD': 'smoke_tests', 'SUBSHARD': '100_99'},
+      );
+      expect(result.exitCode, 1);
+      expect(result.stdout, contains('Invalid subshard name'));
+    });
+  });
 }