tests: don't change hard limit in test-rlimit-util

tests: don't change hard limit in test-rlimit-util

Fixes:
$ make valgrind-tests TESTS=test-rlimit-util
...
Running test-rlimit-util
Assertion 'setrlimit(RLIMIT_NOFILE, &new) >= 0' failed at src/test/test-rlimit-util.c:41, function main(). Aborting.

See http://valgrind.org/docs/manual/dist.news.old.html

The setrlimit system call now simply updates the emulated limits as best
as possible - the hard limit is not allowed to move at all and just
returns EPERM if you try and change it.