t_setrlimit: Adjust resource limit to 20M

With ASLR enabled by default, RLIMIT_STACK test fails due to the fact
that default stack gap can be as big as 15M. Because of that the
resource limit of 4M results in test program receiving SIGSEGV
immediately after exiting the setrlimit syscall. Since the idea of this
test is to check if rlim_cur does not extend past rlim_max, adjusting
the resource limit to 20M should not invalidate the test results.

Obtained from: Semihalf
Sponsored by: Alstom Group
Differential revision: https://reviews.freebsd.org/D33116