Add unit test for method ExternalKey.PubKey #3657

Job	Run time
Compilation check	55s
Docker build check	3m 46s
Format check	24s
cross compilation	5m 14s
run itests	10m 48s
run itests postgres	19m 13s
Lint check	2m 51s
migration version check	10s
go mod check	21s
RPC check	1m 11s
run LiT tests	48s
sample configuration check	46s
Sqlc check	35s
test vector check	1m 9s
run unit tests (unit-race)	12m 31s
run unit tests (unit-cover)	3m 20s
run unit tests (unit dbbackend=postgres)	6m 2s
finish	2s
	1h 10m 6s

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