Fix integration tests bugs introduced during the conversion

packages/path_provider/path_provider_macos/example/integration_test/path_provider_test.dart

@@ -30,16 +30,30 @@ void main() {
     _verifySampleFile(result, 'applicationSupport');
   });
 
+  testWidgets('getLibraryDirectory', (WidgetTester tester) async {
+    final PathProviderPlatform provider = PathProviderPlatform.instance;
+    final String result = await provider.getLibraryPath();
+    _verifySampleFile(result, 'library');
+  });
+
   testWidgets('getDownloadsDirectory', (WidgetTester tester) async {
     final PathProviderPlatform provider = PathProviderPlatform.instance;
     final String result = await provider.getDownloadsPath();
-    _verifySampleFile(result, 'downloads');
+    // Create if necessary, since the native implementation doesn't guarantee
+    // that the folder exists.
+    _verifySampleFile(result, 'downloads', createDirectory: true);
   });
 }
 
 /// Verify a file called [name] in [directoryPath] by recreating it with test
 /// contents when necessary.
-void _verifySampleFile(String directoryPath, String name) {
+///
+/// If [createDirectory] is true, the directory will be created if missing.
+void _verifySampleFile(
+  String directoryPath,
+  String name, {
+  bool createDirectory = false,
+}) {
   final Directory directory = Directory(directoryPath);
   final File file = File('${directory.path}${Platform.pathSeparator}$name');
 
@@ -48,6 +62,10 @@ void _verifySampleFile(String directoryPath, String name) {
     expect(file.existsSync(), isFalse);
   }
 
+  if (createDirectory) {
+    directory.createSync(recursive: true);
+  }
+
   file.writeAsStringSync('Hello world!');
   expect(file.readAsStringSync(), 'Hello world!');
   expect(directory.listSync(), isNotEmpty);