Added unit tests for `go/cmd/rulesctl/` package (#15028) #506

Job	Run time
Build and push vitess/lite Docker images (latest)	2m 58s
Build and push vitess/lite Docker images (mysql57)	3m 8s
Build and push vitess/lite Docker images (mysql80)	3m 3s
Build and push vitess/lite Docker images (percona57)	3m 2s
Build and push vitess/lite Docker images (percona80)	3m 20s
	15m 31s

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