fix(test): use fakeroot option for fuse2fs mount

The ext2 root directory is owned by root, so a non-root user cannot
write to the mounted filesystem. Adding `fakeroot` to the mount options
makes fuse2fs treat the calling user as root for permission checks.

https://claude.ai/code/session_01LfpnUZrgq93MVZgA3KVqE6

Author: claude

tests/one_file_system.rs

@@ -178,7 +178,7 @@ fn cross_device_excludes_mount() {
     let mount_output = Command::new("fuse2fs")
         .with_arg(&image_path)
         .with_arg(&mount_point)
-        .with_args(["-o", "rw"])
+        .with_args(["-o", "rw,fakeroot"])
         .with_stdout(Stdio::piped())
         .with_stderr(Stdio::piped())
         .output()