In replsettest.js, initiateWithAnyNodeAsPrimary:

1. Call replSetInitiate on one node with a one-node config
2. Call getPrimary(), which initializes self._slaves
3. Call replSetReconfig in a loop to add remaining nodes one at a time
4. Call this.awaitSecondaryNodes(self.kDefaultTimeoutMS, self._slaves, 25 /* retryIntervalMS */);
5. In awaitSecondaryNodes, call isMaster on each node in "slaves". Repeat until all slave nodes are secondaries/arbiters.

If there's an election any time after Step 3, then one of the members of self._slaves could be a primary now. However, so awaitSecondaryNodes keeps trying the same set of nodes until it times out. 

Observed in replsettest_control_12_nodes.js. It's probably more common now for a machine to get overloaded, causing heartbeat timeouts and elections:

1. The test starts 12 nodes, the upper limit
2. The nodes are all started in parallel after SERVER-43772
3. There is more time spent in step 3 now that SERVER-45079 requires we add one member at a time