tests/integration: mount /proc in namespaces

tests/integration: mount /proc in namespaces

Because of the use of a PID namespace, we must mount /proc into the
appropriate namespace. We don't do that directly when creating
namespaces as clone() doesn't account for the namespace change with
setns() when we are still in the same process. We also fork a process to
do the mount as it seems mount() doesn't get that we are in a different
mount namespace either. Obviously, setns() has some drawbacks we need to
workaround.

We also mount /proc in the chroot. It's absolutely not safe to do so,
but that's only for address sanitizer to work as expected.