PREPARE stage does not support ava tests

[gibson042]

yarn install doesn't run until install_deps.sh, which doesn't run (or even appear) until the EXECUTE stage (after PREPARE). As a result, prepare.sh scripts cannot call out to e.g. yarn ava initial.test.js for verifying pre-upgrade state unless they also manually yarn install or similar.

[turadg]

The rationale for that decision was to rely on each previous proposal to assert its state. E.g. if proposal N+1 needs some guarantee about proposal N, add it in the post-test for N.

If we can get by with that for a while I think it lowers the need for changing the stage logic.