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.
This was a missing test case from the old P2P tests removed in #5453, which makes sure that all nodes are able to peer with each other regardless of how they discover peers.
Fixes#2795, since the default CI testnet uses a combination of (partially meshed) persistent peers and PEX-based seed nodes.
Erik Grinaker