mod+itest: make custom channel itest flakes less annoying

[guggero]

Runs the litd/terminal only itests separately from the custom channel itests, so flakes can be seen more clearly and ignored if not relevant.

[ellemouton]

nice! thank you!

[ViktorT-11]

Awesome thanks, LGTM code wise 🚀🔥!

Though, I think we need to adjust the "Required Status Checks" before merging this, so that it reflects the new CI jobs. Currently we still get extra pending CI "duplicate jobs":

integration test (bbolt, dbbackend=bbolt)
integration test (postgres, dbbackend=postgres)
integration test (sqlite, dbbackend=sqlite)

As pending checks in the CI job list, as the required checks don't reflect the new updates. I think I lack the required permissions to change that though, so maybe you could do that before merging this @guggero :)?

[guggero]

@ViktorTigerstrom yes, that's correct. I'll need to update them. But only jobs on master can be chosen, so this has to be bypass-merged and then I can update the rules.

[guggero]

@ViktorTigerstrom okay, I updated the required steps. I decided to not include the custom channels one, since it's still flakey. Will include it as required once it's more stable (hopefully soon).

[ViktorT-11]

Awesome, thanks 🎉!