Add "k3s certificate check" clause for better test coverage #9008

Job	Run time
build / Build	6m 58s
Integration Tests (certrotation)	1m 55s
Integration Tests (cacertrotation)	1m 37s
Integration Tests (etcdrestore)	1m 51s
Integration Tests (localstorage)	1m 46s
Integration Tests (startup)	6m 48s
Integration Tests (custometcdargs)	48s
Integration Tests (etcdsnapshot)	7m 7s
Integration Tests (kubeflags)	6m 52s
Integration Tests (longhorn)	2m 37s
Integration Tests (secretsencryption)	2m 45s
Integration Tests (flannelnone)	1m 33s
	42m 37s

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