The purpose of this PR is two-fold: 1) Add breaking code that checks whether the project is known to be core24, to fail early. Otherwise, if the project has a different base (or if there's no project) then we fallback to the current codepath as usual. 2) Add a set of core24 spread tests, each with an expected failure. A lot of the changes are copied over from existing tests, so some commits can mostly be skimmed. I tried to split them up in a way that makes functional sense (to me at least): 1) First commit has the actual feature of checking the base and adding a new RuntimeError; 2) Second commit adds core24 spread tests, but as a straight-up copy of the existing tests in `tests/spread/core22/`. These tests are unchanged and still have `core22` as base, so they don't need a big review; 3) Third commit has the mostly-automated change of base of the core24 tests. I think it's worth a look over because I had to do a few manual changes; 4) The fourth commit adds all expected failures in these new tests because none of them passes cleanly. For example, all tests that call `snapcraft pack` are expected to fail with `"pack" command is not implemented for core24`. The tests now check for this message and exit. 5) Finally, the fifth commit restricts the CI spread runs to only `tests/spread/core24`. I confirmed that the other tests are passing, and restricting this feature-branch to only core24 tests speeds up the development loop considerably.