chainntnfs: fix test `testSingleConfirmationNotification` #17081

Job	Run time
RPC and mobile compilation check	8m 26s
check commits	5m 23s
cross compilation (i386, freebsd-386 linux-386 windows-386)	7m 30s
Sqlc check	1m 9s
lint code	11m 43s
sample configuration check	2m 28s
cross compilation (amd64, darwin-amd64 freebsd-amd64 linux-amd64 netbsd-amd64 openbsd-amd64 windo...	15m 37s
cross compilation (arm, darwin-arm64 freebsd-arm linux-armv6 linux-armv7 linux-arm64 windows-arm)	14m 13s
run unit tests (btcd unit-cover)	19m 35s
run unit tests (unit tags="kvdb_etcd")	21m 11s
basic itests	0s
check pinned dependencies (google.golang.org/grpc v1.59.0)	8s
run unit tests (unit tags="kvdb_postgres")	20m 26s
check pinned dependencies (github.com/golang/protobuf v1.5.3)	10s
run unit tests (unit tags="kvdb_sqlite")	18m 11s
run unit tests (btcd unit-race)	39m 56s
itests	0s
run unit tests (unit-module)	1m 7s
macOS itest	0s
windows itest	0s
check release notes updated	0s
finish	3s
	3h 7m 16s

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