test-skeleton.c: Do not set RLIMIT_DATA [BZ #19648]

With older kernels, it is mostly ineffective because it causes malloc
to switch from sbrk to mmap (potentially invalidating malloc testing
compared to what real appliations do).  With newer kernels which
have switched to enforcing RLIMIT_DATA for mmap as well, some test
cases will fail in an unintended fashion because the limit which was
set previously does not include room for all mmap mappings.

(cherry picked from commit 90005602)
(cherry picked from commit e6eea05e)