Test: improve PFCOUNT with multiple keys testing.
Test: improve PFCOUNT with multiple keys testing.

An user raised a question about a given behavior of PFCOUNT. Added a
test to show the behavior (union) is correct when most of the items are
in common.

dreamquster/redis,ErikDubbelboer/redis,spinlock/redis,soveran/redis,soveran/redis,neomantra/redis,izhoujie/redis,h0x91b/redis,kensou97/redis,shining-yang/redis,neomantra/redis,pmem/redis,soloestoy/redis,tanghaodong25/redis,GitHubMota/redis,ton31337/redis,GitHubMota/redis,hgl888/redis,hgl888/redis,ofirluzon/redis,yuhc/redis-benchmark-enhanced,sunheehnus/redis,arijitvt/redis,itamarhaber/redis,GitHubMota/redis,ctripcorp/redis,soloestoy/redis,charsyam/redis,soveran/redis,antirez/redis,nnog/redis,ErikDubbelboer/redis,YongMan/redis,OmarQunsul/graph-redis,kensou97/redis,Markgorden/redis,shreesundara/redis,ofirluzon/redis,ton31337/redis,izhoujie/redis,tjschuck/redis,shining-yang/redis,sunheehnus/redis,yossigo/redis,pmem/redis,hgl888/redis,ctripcorp/redis,ofirluzon/redis,PKRoma/redis,soloestoy/redis,linfangrong/redis,antirez/redis,tjschuck/redis,OmarQunsul/graph-redis,weizijun/redis,spinlock/redis,Sciumo/redis,PKRoma/redis,oranagra/redis,Markgorden/redis,Sciumo/redis,PKRoma/redis,yossigo/redis,ctripcorp/redis,kensou97/redis,spinlock/redis,tanghaodong25/redis,weizijun/redis,rogerchina/redis,YongMan/redis,oranagra/redis,arijitvt/redis,shreesundara/redis,PKRoma/redis,ErikDubbelboer/redis,antirez/redis,weizijun/redis,nnog/redis,h0x91b/redis,izhoujie/redis,charsyam/redis,PKRoma/redis,tanghaodong25/redis,badboy/redis,itamarhaber/redis,yuhc/redis-benchmark-enhanced,zguangyu/redis,MasahikoSawada/redis,dreamquster/redis,hgl888/redis,YongMan/redis,zguangyu/redis,h0x91b/redis,ofirluzon/redis,linfangrong/redis,tanghaodong25/redis,badboy/redis,MasahikoSawada/redis,arijitvt/redis,rouzier/redis,rouzier/redis,oranagra/redis,colstrom/redis,sunheehnus/redis,zguangyu/redis,weizijun/redis,ctripcorp/redis,shining-yang/redis,MasahikoSawada/redis,shining-yang/redis,rogerchina/redis,linfangrong/redis,neomantra/redis,dreamquster/redis,izhoujie/redis,yossigo/redis,pmem/redis,colstrom/redis,nnog/redis,tjschuck/redis,soveran/redis,YongMan/redis,Markgorden/redis,yossigo/redis,tjschuck/redis,neomantra/redis,rouzier/redis,colstrom/redis,yuhc/redis-benchmark-enhanced,h0x91b/redis,charsyam/redis,yuhc/redis-benchmark-enhanced,colstrom/redis,rouzier/redis,zguangyu/redis,ErikDubbelboer/redis,charsyam/redis,sunheehnus/redis,rogerchina/redis,antirez/redis,shreesundara/redis,Sciumo/redis,soloestoy/redis,oranagra/redis,OmarQunsul/graph-redis,itamarhaber/redis,Sciumo/redis,charsyam/redis,spinlock/redis,dreamquster/redis,pmem/redis,yossigo/redis,badboy/redis,ton31337/redis,soloestoy/redis,badboy/redis,arijitvt/redis,MasahikoSawada/redis,oranagra/redis,nnog/redis,neomantra/redis,itamarhaber/redis,shreesundara/redis,linfangrong/redis,ofirluzon/redis,OmarQunsul/graph-redis,ton31337/redis,Markgorden/redis,GitHubMota/redis,kensou97/redis,rogerchina/redis