go/worker/keymanager: Speed up secret generation in tests

[peternose]

Description
When a key manager node is the sole member of the committee, it generates master and ephemeral secrets immediately rather than at a random block within an epoch. In tests, this allows us to transition to the next epoch earlier, as soon as all required secrets are generated.

Tests
This change can be seen in the logs, when running a single key manager node:

{"caller":"worker.go:1136","epoch":1,"height":30,"level":"debug","module":"worker/keymanager","msg":"block height for generating secrets selected","ts":"2023-10-31T19:34:45.56695587Z"}
{"caller":"worker.go:635","epoch":2,"generation":0,"height":22,"level":"info","module":"worker/keymanager","msg":"generating master secret","ts":"2023-10-31T19:34:46.973834368Z"}

Observe that the secret was generated at height 22, and not at height 30.