raft: Fix election "logs converge" test
raft: Fix election "logs converge" test

The "logs converge" case in TestLeaderElectionPreVote was incorrectly
passing because some nodes were not actually using the preVoteConfig.
This test case was more complex than its siblings and it was not
verifying what it wanted to verify, so pull it out into a separate test
where everything can be tested more explicitly.

Fixes #6895

a-robinson/etcd,lumjjb/etcd,yudai/etcd,handsomesun/etcd,tguangch/etcd,YuleiXiao/etcd,sebrandon1/etcd,bluebreezecf/etcd,ajityagaty/etcd,armstrongli/etcd,mkumatag/etcd,akauppi/etcd,rnaveiras/etcd,heyitsanthony/etcd,vimalk78/etcd,bdarnell/etcd,li-ang/etcd,sinsharat/etcd,cockroachdb/etcd,jlamillan/etcd,YuleiXiao/etcd,CrazyJvm/etcd,xiang90/etcd,sinsharat/etcd,yudai/etcd,ajityagaty/etcd,qqsun8819/etcd,glevand/coreos--etcd,etcd-io/etcd,cockroachdb/etcd,mqliang/etcd,fanminshi/etcd,bluebreezecf/etcd,mqliang/etcd,whuwxl/etcd,xiang90/etcd,hongchaodeng/etcd,dannysauer/etcd,lorneli/etcd,plasticbox/etcd,visheshnp/etcd,fanminshi/etcd,lumjjb/etcd,plasticbox/etcd,glevand/coreos--etcd,tguangch/etcd,qqsun8819/etcd,ravigadde/etcd,bdarnell/etcd,insequent/etcd,rchicoli/etcd,rchicoli/etcd,sebrandon1/etcd,armstrongli/etcd,whuwxl/etcd,vimalk78/etcd,heyitsanthony/etcd,hongchaodeng/etcd,a-robinson/etcd,ajityagaty/etcd,handsomesun/etcd,joshix/etcd,akauppi/etcd,visheshnp/etcd,mkumatag/etcd,gyuho/etcd,dannysauer/etcd,coreos/etcd,joshix/etcd,rnaveiras/etcd,mqliang/etcd,etcd-io/etcd,CrazyJvm/etcd,gyuho/etcd,lorneli/etcd,rnaveiras/etcd,etcd-io/etcd,li-ang/etcd,insequent/etcd,mkumatag/etcd,ravigadde/etcd,mickep76/etcd,jlamillan/etcd,mickep76/etcd,coreos/etcd