Comment 8 for bug 1978785

It turns out our existing manager test was testing the slightly wrong thing - it simulated only that both the kernel and the base fail at the same time. I added a test that tests this issue now in https://github.com/snapcore/snapd/commit/6a260bc5cdf7452d19f087214b4484e84e17cfab and I see the problematic behavior with that now too - i.e. that the "base" is set to try and the new core20 that is still set as base got removed from the system.