storage: add more tests for synced, unsynced for watchable store
storage: add more tests for synced, unsynced for watchable store

This adds more tests on functions that updates synced and unsynced in watchable
store. Preparatory change for https://github.com/coreos/etcd/issues/3848.

li-ang/etcd,armstrongli/etcd,jonboulle/etcd,achanda/etcd,swingbach/etcd,cchamplin/etcd,StorReduce/etcd,a-robinson/etcd,plasticbox/etcd,ravigadde/etcd,handsomesun/etcd,wantonsolutions/dinv-etcd,mkumatag/etcd,bluebreezecf/etcd,xiang90/etcd,sinsharat/etcd,li-ang/etcd,StorReduce/etcd,swingbach/etcd,ajityagaty/etcd,etcd-io/etcd,plasticbox/etcd,whuwxl/etcd,glevand/coreos--etcd,YuleiXiao/etcd,sinsharat/etcd,sofuture/etcd,glycerine/etcd,tamird/etcd,xiang90/etcd,mqliang/etcd,dnaeon/etcd,siddontang/etcd,OyTao/etcd_learning,a-robinson/etcd,zaphod-concur/etcd,nekto0n/etcd,dnaeon/etcd,insequent/etcd,coreos/etcd,purpleidea/etcd,jlamillan/etcd,gyuho/etcd,ravigadde/etcd,mkumatag/etcd,coreos/etcd,bluebreezecf/etcd,akauppi/etcd,westhood/etcd,qqsun8819/etcd,etcd-io/etcd,linuxcer/etcd,rchicoli/etcd,visheshnp/etcd,ktateish/etcd,bdarnell/etcd,sayden/etcd,sayden/etcd,armstrongli/etcd,raoofm/etcd,Timer/etcd,OyTao/etcd_learning,luxas/etcd,cchamplin/etcd,CrazyJvm/etcd,rnaveiras/etcd,vimalk78/etcd,visheshnp/etcd,yudai/etcd,cockroachdb/etcd,jlamillan/etcd,sofuture/etcd,nekto0n/etcd,sebrandon1/etcd,dannysauer/etcd,hongchaodeng/etcd,vimalk78/etcd,lorneli/etcd,tguangch/etcd,mkumatag/etcd,ajityagaty/etcd,glevand/coreos--etcd,YuleiXiao/etcd,joshix/etcd,handsomesun/etcd,yudai/etcd,shawnps/etcd,ktateish/etcd,glycerine/etcd,whuwxl/etcd,lumjjb/etcd,siddontang/etcd,zaphod-concur/etcd,mqliang/etcd,lumjjb/etcd,wantonsolutions/dinv-etcd,fanminshi/etcd,raoofm/etcd,CrazyJvm/etcd,westhood/etcd,jonboulle/etcd,lorneli/etcd,mqliang/etcd,heyitsanthony/etcd,chipironcin/etcd,rnaveiras/etcd,endocode/etcd,chipironcin/etcd,ajityagaty/etcd,endocode/etcd,joshix/etcd,heyitsanthony/etcd,mickep76/etcd,hongchaodeng/etcd,cockroachdb/etcd,achanda/etcd,gyuho/etcd,Timer/etcd,fanminshi/etcd,rchicoli/etcd,wantonsolutions/dinv-etcd,sebrandon1/etcd,rnaveiras/etcd,bdarnell/etcd,mickep76/etcd,tguangch/etcd,qqsun8819/etcd,tamird/etcd,linuxcer/etcd,dannysauer/etcd,purpleidea/etcd,etcd-io/etcd,akauppi/etcd,luxas/etcd,insequent/etcd,shawnps/etcd