test: fix handling of start height in generated E2E testnets (#5563)
In #5488 the E2E testnet generator changed to setting explicit `StartAt` heights for initial nodes. This broke the runner, which expected all initial nodes to have `StartAt: 0`, as well as validator set scheduling in the generator. Testnet loading now normalizes initial nodes to have `StartAt: 0`.
This also tweaks waiting for misbehavior heights to only use an additional wait if there actually is any misbehavior in the testnet, and to output information when waiting.