tests: check decoding of modify_ldt with for 4-byte-available user_desc

tests: check decoding of modify_ldt with for 4-byte-available user_desc

Since there is possibility now that strace read only entry_number field
of the user_desc struct, let's check that it doesn't do it with
modify_ldt syscall.

* tests/modify_ldt.c (main): Add 4-byte-sized tail_alloc'ed variable.
Try to provide it as an argument to modify_ldt.