Create web tests suite & runner_utils (#146592)

Move from `test.dart` into new `runner_utils.dart` file:
- shard calculation related methods + `shuffleSeed` logic
- env variable flag calculation
- runner wrapper methods `runDartTest` and `_runFromList`

Create web tests suite class containing:
- web long running tests
- runWebHtmlUnitTests
- runWebCanvasKitUnitTests
- runWebSkwasmUnitTests
- the associate helper methods with the above

Author: sealesj

dev/bots/suite_runners/run_flutter_packages_tests.dart

@@ -9,7 +9,6 @@ import 'package:file/local.dart';
 import 'package:path/path.dart' as path;
 
 import '../run_command.dart';
-import '../test.dart';
 import '../utils.dart';
 
 /// Executes the test suite for the flutter/packages repo.