Reduce need for checked-in keys in repo

[jsha]

trafficstars

We have some keys that we check in to the repo for test purposes. Since it occasionally happens that someone grabs one of those keys and tries to use it for real(!) we'd like to use fewer of them, and in particular avoid producing new ones (since the existing ones are blocked already).

Specifically for test/grpc-creds/ and test/redis-tls we have a couple of options:

• Modify minica to support reusing the same key for all end-entity certificates
• Modify minica to make ecdsa keys (so it's faster) and generate keys and certs on demand like we do with the issuance hierarchy

[pgporada]

There's this PR that will fix the second bullet point.

[jsha]

That PR is only for roots; we'd need some additional work for end-entity certificates.

[aarongable]

Subsumed by https://github.com/letsencrypt/boulder/issues/7476