tests/shell: add "-S|--setup-host" option to set sysctl for rootless tests

tests/shell: add "-S|--setup-host" option to set sysctl for rootless tests

Most tests can run just fine without root. A few of them will fail if
/proc/sys/net/core/{wmem_max,rmem_max} is too small (as it is by default
on the host).

The easy workaround is to bump those limits once. This has to be
repeated after each reboot.

Doing that manually (every time) is cumbersome. Add a "--setup-host"
option for that.

Usage:

  $ sudo ./tests/shell/run-tests.sh -S
  Setting up host for running as rootless (requires root).
      echo 4096000 > /proc/sys/net/core/rmem_max (previous value 100000)
      echo 4096000 > /proc/sys/net/core/wmem_max (previous value 100000)

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>