selftests: forwarding: lib: Add sysctl_set(), sysctl_restore()

Add two helper functions: sysctl_set() to change the value of a given
sysctl setting, and sysctl_restore() to change it back to what it was.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>