#!/bin/bash
awk -F '"' 'BEGIN { OFS=FS }
{ for(i=2; i<=NF; i+=2)
gsub(/,/, "", $i) }1'
IyEvYmluL2Jhc2gKYXdrIC1GICciJyAnQkVHSU4geyBPRlM9RlMgfQogIHsgZm9yKGk9MjsgaTw9TkY7IGkrPTIpCiAgICBnc3ViKC8sLywgIiIsICRpKSB9MSc=
MSwyLDMsJnF1b3Q7dGVzdDEsdGVzdDIsdGVzdDMmcXVvdDssNCw1LDY=
1,2,3,"test1,test2,test3",4,5,6