Make chutney check for relay microdescriptors before verifying

In #33232 (moved), we make chutney check for relay and bridge descriptors, and relays in the consensus, before verifying. But we don't check for relay microdescriptors.

The easiest way to check for relay microdescriptors, is using each relay's ed25519 key. In #30642 (moved), we will add an ed25519-identity file to each relay's data directory. Chutney can read the contents of this file, and then search for it in cached-microdescriptors and cached-microdescriptors.new.