Add required tests for `go/netutil` (#15392) #503

Job	Run time
Build and push vitess/lite Docker images (latest)	3m 7s
Build and push vitess/lite Docker images (mysql57)	3m 6s
Build and push vitess/lite Docker images (mysql80)	3m 11s
Build and push vitess/lite Docker images (percona57)	2m 50s
Build and push vitess/lite Docker images (percona80)	3m 28s
	15m 42s

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