[testnet] Test remote compatibility on the testnet branch

[ma2bd]

Motivation

Test that we don't break clients (and to some degree validators) on the testnet branch

Proposal

Run the e2e tests against the existing testnet in CI

Future work:

• An alternative approach would to set up a local network (validator + faucet) with exactly the same commit as the production environment. This could be done in different ways but requires more design.
• It would be nice to also hosted web applications somehow.

Test Plan

CI