Btrfs at least supports 16 EiB files (limited in practice to 8EiB by the
Linux VFS code which uses signed 64-bit offsets). So fix the fs.zig test
case to expect either a FileTooBig or success from truncating a file to
8EiB. And test that beyond that size the offset is interpreted as a
negative number.
Fixes#24242
710632b45c