tests: Fix race condition in aslr_setuid

Use a cloexec pipe to block the parent until the child is ready.

While here, redirect the output from ping to /dev/null, and mark the
test as requiring the inet feature since we ping the IPv4 loopback.

PR: 296116
MFC after: 1 week
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D57734