Test suite reusability

[0xmichalis]

Ensure the existing tests that exercise various e2e scenarios for a grin wallet are reusable enough to run against a standalone wallet. Once that's done, we should be able to easily test various upgrade scenarios, eg. by running the e2e suite on a new wallet with old data or test sending transactions between wallets that support different slate versions.

Such a reusable e2e test suite should also be beneficial to other wallet implementations.