compose_box [nfc]: On Windows, skip a test that fails.

The `attach from camera` test fails on
Windows due to the path separator
used in the split function of the XFile.name
getter in `cross_file` being '\' instead of '/'.
This results in the getter returning the
entire path on Windows instead of the
last word present after the last '/' in the path.

Read this excellent explanation by
@chrisbobbe for better understanding:
https://chat.zulip.org/#narrow/channel/516-mobile-dev-help/topic/failing.20composebox.20test.20while.20running.20check.20script/near/2000301

Author: apoorvapendse

test/widgets/compose_box_test.dart

@@ -1,5 +1,6 @@
 import 'dart:async';
 import 'dart:convert';
+import 'dart:io';
 
 import 'package:checks/checks.dart';
 import 'package:file_picker/file_picker.dart';
@@ -581,7 +582,12 @@ void main() {
       });
 
       // TODO test what happens when capturing/uploading fails
-    });
+
+      // This test fails on Windows due to the Platform.pathSeparator.
+      // used in the split function of the XFile.name getter in cross_file being '\' instead of '/'.
+      // This results in the getter returning the entire path on Windows instead of the
+      // last word present after the last '/' in the path.
+    }, skip: Platform.isWindows);
   });
 
   group('error banner', () {