mod+itest: make custom channel itest flakes less annoying #2416

Job	Run time
check commits	4m 19s
frontend tests on ubuntu-latest	2m 19s
cross compilation (i386, linux-386)	4m 53s
backend build on ubuntu-latest	10m 25s
frontend tests on windows-latest	13m 42s
integration test (bbolt, itest icase=terminal dbbackend=bbolt)	10m 51s
cross compilation (amd64, darwin-amd64 linux-amd64 windows-amd64)	13m 25s
backend build on windows-latest	24m 17s
integration test (sqlite, itest icase=terminal dbbackend=sqlite)	10m 24s
frontend tests on macOS-latest	2m 4s
backend build on macOS-latest	9m 54s
cross compilation (arm, darwin-arm64 linux-armv6 linux-armv7 linux-arm64)	17m 35s
integration test (postgres, itest icase=terminal dbbackend=postgres)	11m 1s
integration test (custom-channels, itest-only icase=custom_channels)	16m 47s
lint	5m 35s
RPC proto compilation check	4m 31s
check release notes updated	2s
Sqlc check	41s
run unit tests (unit-race)	6m 49s
run unit tests (unit)	5m 1s
run unit tests (unit dbbackend=postgres)	5m 22s
run unit tests (unit dbbackend=sqlite)	4m 44s
	3h 4m 41s

Provide feedback