Summary:
(Copied from the description of D35173 / `9951cd149838587f96cc40cb77859fa69322ab0a`, which
fixed this issue for MiniCluster tests.)

ExternalMiniCluster tests currently rely on a RAFT failure (`ReportFailDetected`) for the master leader election on startup. This wastes 3s on each ExternalMiniCluster test. This diff adds code to trigger an election in ExternalMiniCluster::StartMasters() instead.
This is done at the ExternalMiniCluster layer (and not by RAFT) because ExternalMiniCluster knows when all peers have been started, and thus when an election should succeed.
Jira: DB-15538

Test Plan: existing tests

Reviewers: hsunder

Reviewed By: hsunder

Subscribers: yql, ybase

Differential Revision: https://phorge.dev.yugabyte.com/D42166