Add globalSetup and addGlobalTearDown APIs for cross-suite setup and teardown (#2714)

Adds `globalSetup()` and `addGlobalTearDown()` APIs in `package:test_api` / `package:test` to execute shared global setup scripts once on the host test runner, memoizing results across all test suites and executing registered teardowns when the test runner closes.

Addresses #2225

The main motivation is to support setup shared across multiple test suites (`_test.dart` files). Examples include:
* Precompiling a binary used by multiple end-to-end tests
* Provisioning test databases or starting local test servers

### Example
```dart
// test/setup_server.dart
import 'dart:io';
import 'package:test/global.dart';

Future<Map<String, dynamic>> setUp() async {
  var server = await HttpServer.bind('localhost', 0);
  addGlobalTearDown(() async => await server.close(force: true));
  return {'port': server.port};
}

// test/service_test.dart
import 'package:test/test.dart';

void main() {
  test('connects to server', () async {
    final config = await globalSetup(
      Uri(path: '/test/setup_server.dart'),
    ) as Map;
    final port = config['port'] as int;
    // ...
  });
}
```

### Key Features
* **`globalSetup(Uri uri)`**:
  * Runs the Dart file at `uri` in an isolate on the host runner and returns its JSON-encodable result.
  * Memoizes the JSON-encoded result so the script only executes once per canonical URI, while returning an independent decoded value to each caller.
  * Must be called from within a running test or a `setUp()` / `setUpAll()` callback.
  * Bootstraps isolates via an in-memory `data:` URI using `Isolate.spawnUri` (sharing architecture with `spawnHybridUri`), avoiding filesystem compilation or runner-specific compiler hooks.
* **`addGlobalTearDown(FutureOr<void> Function() callback)`** (exported from `package:test/global.dart`):
  * Called from within a setup script to register cleanup tasks.
  * Executed in reverse registration order (LIFO) when the test runner closes.
  * Best-effort cleanup: all registered teardowns run even if an earlier teardown throws, with errors reported to stderr and runner exiting with code 1.
  * 30-second per-isolate timeout to prevent hanging teardowns.
* **URI Resolution**:
  * **Root-relative** (`Uri(path: '/test/setup.dart')`): Resolved relative to package root (`pubspec.yaml`).
  * **`package:` URIs** (`Uri.parse('package:my_pkg/test_helpers.dart')`): Resolved synchronously via package config.
  * **Relative URIs** (`Uri(path: 'setup.dart')` or `Uri(path: '../setup.dart')`): Resolved relative to the executing test suite's directory.
  * **`file:` URIs**: Resolved as absolute filesystem paths.
  * Fragments are automatically removed for consistent compilation caching. Relative, root-relative, and file URIs strictly prohibit query parameters.
* **Cross-Platform Support**:
  * Dispatches over `test_channel`, allowing browser, Node, and VM tests alike to trigger host-side setup.
* **Standalone Execution Support**:
  * Tests run directly via `dart <test_file>.dart` execute setup scripts using a standalone scaffolding fallback and run teardowns on process exit.
* **Flutter Compatibility**:
  * Works out of the box with `flutter test` without runner-level code changes.