tests: parameterize gadget snap channel

[fgimenez]

This will allow us to create spread-cron branches to watch the promotions of the gadget snap and trigger spread executions when changes are detected.

[pedronis]

I find this problematic because at this point we aren't using the asserted firstboot installation path for anything, we should really fix ubuntu-image to support per snap channels :/

[pedronis]

as I proposed in standup what we could do is use ubuntu-image --channel=...
directly and not "snap download"
if all CHANNEL env var are the same

[fgimenez]

@pedronis thanks, please take a look

[chipaca]

Looks alright, with one nitpicky request, and a question

[chipaca]

the quoting shenanigans going on here are valid, but best reserved for when they're needed. Here they're not: can you rewrite these like

EXTRA_FUNDAMENTAL="--extra-snaps $PWD/pc-kernel_*.snap"

?

[fgimenez]

Done, thanks!

[chipaca]

am I wrong in understanding that this means you can't have both gadget and kernel set to non-edge unless they're both set to the same thing, otherwise the gadget gets forced to edge? Is that intended?

[fgimenez]

Yes, this is meant to be used from spread-cron after detecting a new fundamental snap has been promoted to any of the channels, only one of them should be different from the default image channel (currently edge).

[fgimenez]

@chipaca pls take another look, with the same premises i've updated the case(s) when kernel and gadget have different channels so that the gadget channel is not forced, also the gadget snap is never downloaded (aiming to address @pedronis concerns).