fix(spread): update tests that manipulate architectures
This reflects a change in behavior in core24:
- Trying to build multiple snaps in a single run is an error;
- Trying to build no snaps at all is also an error.
Some of the subtests in architectures/ are still failing, but that's because
they need other fixes (like SNAPCRAFT_BUILD_FOR and "build-for: all" support).
In addition, the build-for-all test is removed because we decided to _not_
support "build-for: all" for now in core24, because the concept itself of
multiplatform snaps needs to be further discussed.
This commit adds support for extensions in core24 runs. This takes two changes:
- The extension-related commands (list- and expand-) are sort of "ported over"
to craft-application's AppCommand base class. The commands for core22 and
core24 now share the implementation.
- SnapcraftApplication is updated to actually apply the extensions when loading
the project. Since we don't have any core24-capable extensions yet, this is
verified at the regular-test level with an integration-ish test that runs the
application and verifies that the dummy test extension is applied.
