Beat itest [2/3]: document and fix itest flakes #16870

Job	Run time
check commits	13m 0s
RPC and mobile compilation check	3m 28s
cross compilation	30m 25s
lint code	7m 0s
check pinned dependencies (google.golang.org/grpc v1.59.0)	3s
sample configuration check	1m 51s
check pinned dependencies (github.com/golang/protobuf v1.5.3)	4s
Sqlc check	2m 48s
run unit tests (btcd unit-cover)	19m 40s
run unit tests (unit tags="kvdb_etcd")	20m 43s
run macOS itest	0s
run ubuntu itests	0s
run unit tests (unit tags="kvdb_postgres")	20m 3s
run unit tests (unit tags="kvdb_sqlite")	18m 23s
check release notes updated	0s
run unit tests (btcd unit-race)	39m 32s
run windows itest	0s
run unit tests (unit-module)	1m 12s
finish	8s
	2h 58m 20s

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