tests: Fix race condition in aslr_setuid

[des]

Summary

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

[markj]

Thanks.

Have you ever tried to run the test suite without an INET stack?

[des]

In D57734#1323774, @markj wrote:

Have you ever tried to run the test suite without an INET stack?

Bits and pieces. Never the whole thing.