Demote "specialized_init_system" certification test from "essential" to "normal"

[martin-mat]

Specialized_init_system in my opinion does not deserve to be "essential" for following reasons:

• the main reason for having init system inside containers is handling signals and zombies. There are specialized tests already among essential tests for those so this one seem to be redundant. Additionally, specialized init system is just one of ways how to handle signals/zombies.

• there is no good universal check for a specialized init system in cnf-testsuite. Current implementation checks for use just of "tini", "dumb-init", "s6-vscan" which is limiting.

I propose to leave it among certification tests, but demote from "essential" to "normal".