Update downgrade-upgrade test

[bitdivine]

Motivation

The downgrade-upgrade test needs to be able to verify that accounts are preserved across schema updates.

Changes

• Make it possible to specify a wasm to downgrade to. This way the test can be run with a wasm that implements the future schema S0.early and another, undefined.late that is not yet in production.
• Get account data; this was already present but commented out as prod did not have account data in a stable, usable form.

Tests

Todos

• Add entry to changelog (if necessary).

[lmuntaner]

LGTM, thanks!