Add "k3s certificate check" clause for better test coverage #8993

Job	Run time
build / Build	7m 15s
Integration Tests (certrotation)	1m 53s
Integration Tests (cacertrotation)	1m 35s
Integration Tests (etcdrestore)	1m 39s
Integration Tests (localstorage)	1m 39s
Integration Tests (startup)	6m 24s
Integration Tests (custometcdargs)	1m 0s
Integration Tests (etcdsnapshot)	8m 11s
Integration Tests (kubeflags)	6m 51s
Integration Tests (longhorn)	2m 19s
Integration Tests (secretsencryption)	2m 47s
Integration Tests (flannelnone)	1m 16s
	42m 49s

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