Alert key rotation test (WIP)

Some scaffolding to make it easier to rotate alert keys, and to test the new keys. Adds an utility that generates the alert keys (public/private) along with test data.

I don't think that `bitcoin-genalertkey` should be built by default, due to the rarity that this is used. On the other hand maintaining it as part of the repository means that it won't be out of date every time we actually need it (same goes for the utility used to generate and sign alerts).

- Tests: test both mainnet and testnet alert keys

**Note:** this pull request changes the alert keys. Do not merge yet!