Limit drand relay calls in tests

[LesnyRumcajs]

Summary

Unit tests should depend on external resources as little as possible (to reduce flakiness and be nice). Right now a single cargo nextest would make burst of ~23 calls to the drand HTTPS relay

      2 api2.drand.sh
      2 api3.drand.sh
     15 api.drand.sh
      4 drand.cloudflare.com

This doesn't count possible retries.

Completion Criteria

• contrive a way to limit drand relay usage in tests. We already have some mocking mechanisms around there. Doing some memoization might also be fine, but ideally we'd go down to zero foreign calls.

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