overlord: introduce Mock which enables to use Overlord.Settle for settle in many more places (#3856)
overlord.Mock creates a no-manager Overlord supporting AddManager to add a subset of managers, mainly for letting tests use Overlord.Settle even with just 1 or few managers.
Moved calling some global init code into Manager creation functions to avoid unexpected confusing side-effects on tests once they all start to import overlord which imports all managers.
This test is failing case we run with $SPREAD_BACKEND = external because
there are not snapd_*.deb files to copy. Skipping test when there are not -deb files built.
daemon, snapstate: move ensureCore from daemon/api.go into snapstate.go (#3727)
This is prereq work for auto-installing missing base snapd and missing default-provider for content snaps.
The logic to add tasks to install core is added in a 'prerequisites' task that either wait for another change to do that or adds the necessary lane to the current change.