The previous implemention of hybrid set testing, which was entirely my
own creation, was a bit peculiar, and I think this probably clears thins up.
The previous implementation had far fewer legacy nodes in hybrid
networks, *and* also for some reason that I can't quite explain,
caused a test case to fail.
This is just a configuration change to default to using the new stack
unless explicitly disabled (e.g. `UseLegacy`) this renames the
configuration value and makes the configuration logic more clear.
The legacy option is good to retain as a fallback if the new stack has
issues operationally, but we should make sure that most of the time
we're using the new stack.
Closes#5291. Adds a randomized testnet generator. Nightly CI job will be submitted separately. A few of the testnets can be a bit flaky, even after disabling known-faulty behavior and making minor tweaks, and the larger networks may be too resource-intensive to run in CI - this will be optimized separately.