Add missing peer selection tests to Testnet diffusion tests.

Currently [peer selection tests](https://github.com/input-output-hk/ouroboros-network/blob/master/ouroboros-network/sim-tests/Test/Ouroboros/Network/PeerSelection.hs) are more complete and include big ledger peers tests. We need to have these tests for [diffusion](https://github.com/input-output-hk/ouroboros-network/blob/master/ouroboros-network/sim-tests/Test/Ouroboros/Network/Testnet.hs) as well.

* [ ] `prop_governor_target_above` is important because of [this issue](https://github.com/input-output-hk/ouroboros-network/pull/4734#pullrequestreview-1755495877).