Test suite reusability

[0xmichalis]

Ensure the existing tests that exercise various e2e scenarios for a grin node are reusable enough to run against a standalone node. Once that's done, we should be able to easily test various upgrade scenarios, eg. by running the e2e suite on a new node with old data or with older/newer peers.

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