Beat itest [2/3]: document and fix itest flakes #16655

Job	Run time
RPC and mobile compilation check	6m 32s
lint code	7m 4s
Sqlc check	3m 33s
check commits	3m 4s
cross compilation	32m 21s
check pinned dependencies (google.golang.org/grpc v1.59.0)	5s
run macOS itest	0s
check pinned dependencies (github.com/golang/protobuf v1.5.3)	4s
sample configuration check	4m 37s
run ubuntu itests	0s
run unit tests (btcd unit-cover)	16m 22s
check release notes updated	0s
run unit tests (unit tags="kvdb_etcd")	18m 54s
run windows itest	0s
run unit tests (unit tags="kvdb_postgres")	17m 28s
run unit tests (unit tags="kvdb_sqlite")	15m 55s
run unit tests (btcd unit-race)	35m 46s
run unit tests (unit-module)	1m 57s
finish	4s
	2h 43m 46s

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