selftests: forwarding: lib: quote the sysctl values

When set/restore sysctl value, we should quote the value as some keys
may have multi values, e.g. net.ipv4.ping_group_range

Fixes: f5ae5778

 ("selftests: forwarding: lib: Add sysctl_set(), sysctl_restore()")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20230208032110.879205-1-liuhangbin@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>