priority_takeover_one_node_higher_priority jstest needs longer stepDownGuard

The test occasionally fails because the high priority node steps up before the low priority node is elected. We can increase the stepDownGuard in priority_takeover_one_node_higher_priority.js to 10 * electionTimeoutMillis, then explicitly end the stepdown period on the high priority node with replSetFreeze after the low priority node is elected to avoid the test taking longer.