backup_restore_rolling.js can fail due to stepdown

If test machine load is high enough to delay heartbeats by over 10 seconds, a replica can call for an election and cause the primary to step down. BackupRestoreTest doesn't anticipate this and can fail. In one observed case, it fails when calling listDatabases on a replica it thought was still the primary - this results in "not master" and aborts the test.

Let's increase the electionTimeoutMillis when setting up the replica set to prevent elections.