Ensure that integration tests are actually being run (#3857)

Many of our integration tests weren't actually being run on CI because they were in the wrong place and/or missing the driver file, and it seems we've just never noticed. This makes a number of changes:
- Ensures that all packages with integration tests also have a driver file in the right location.
  - Also standardizes the format of those files, as the boilerplate `main()` is available in `integration_test`.
- Ensures that all integration_test directories are in the right place.
  - In a couple of places, removes a duplicate copy of the integration test file.
- Makes it an error for a plugin that's not excluded to not have integration tests, so this can't easily happen again.
- Adds logging of what's being run and skipped, so if something does go wrong in the future it's easy to determine what from the logs.
  - Excludes `*_platform_interface` since the logging was (potentially confusingly) reporting that they were skipped because they don't support the current platform. Skipping them is correct, just not for that reason.
- Excludes the plugins that currently have no integration tests, with references to issues about adding them.

Fixes https://github.com/flutter/flutter/issues/81929

script/tool/lib/src/drive_examples_command.dart

@@ -52,23 +52,37 @@ class DriveExamplesCommand extends PluginCommand {
   @override
   Future<void> run() async {
     final List<String> failingTests = <String>[];
+    final List<String> pluginsWithoutTests = <String>[];
     final bool isLinux = argResults[kLinux] == true;
     final bool isMacos = argResults[kMacos] == true;
     final bool isWeb = argResults[kWeb] == true;
     final bool isWindows = argResults[kWindows] == true;
     await for (final Directory plugin in getPlugins()) {
+      final String pluginName = plugin.basename;
+      if (pluginName.endsWith('_platform_interface') &&
+          !plugin.childDirectory('example').existsSync()) {
+        // Platform interface packages generally aren't intended to have
+        // examples, and don't need integration tests, so silently skip them
+        // unless for some reason there is an example directory.
+        continue;
+      }
+      print('\n==========\nChecking $pluginName...');
+      if (!(await _pluginSupportedOnCurrentPlatform(plugin, fileSystem))) {
+        print('Not supported for the target platform; skipping.');
+        continue;
+      }
+      int examplesFound = 0;
+      bool testsRan = false;
       final String flutterCommand =
           const LocalPlatform().isWindows ? 'flutter.bat' : 'flutter';
       for (final Directory example in getExamplesForPlugin(plugin)) {
+        ++examplesFound;
         final String packageName =
             p.relative(example.path, from: packagesDir.path);
-        if (!(await _pluginSupportedOnCurrentPlatform(plugin, fileSystem))) {
-          continue;
-        }
         final Directory driverTests =
             fileSystem.directory(p.join(example.path, 'test_driver'));
         if (!driverTests.existsSync()) {
-          // No driver tests available for this example
+          print('No driver tests found for $packageName');
           continue;
         }
         // Look for driver tests ending in _test.dart in test_driver/
@@ -160,6 +174,7 @@ Tried searching for the following:
           }
 
           for (final String targetPath in targetPaths) {
+            testsRan = true;
             final int exitCode = await processRunner.runAndStream(
                 flutterCommand,
                 <String>[
@@ -177,6 +192,11 @@ Tried searching for the following:
           }
         }
       }
+      if (!testsRan) {
+        pluginsWithoutTests.add(pluginName);
+        print(
+            'No driver tests run for $pluginName ($examplesFound examples found)');
+      }
     }
     print('\n\n');
 
@@ -188,6 +208,15 @@ Tried searching for the following:
       throw ToolExit(1);
     }
 
+    if (pluginsWithoutTests.isNotEmpty) {
+      print('The following plugins did not run any integration tests:');
+      for (final String plugin in pluginsWithoutTests) {
+        print(' * $plugin');
+      }
+      print('If this is intentional, they must be explicitly excluded.');
+      throw ToolExit(1);
+    }
+
     print('All driver tests successful!');
   }