smoketest: Add environment variable to easily wrap servers
smoketest: Add environment variable to easily wrap servers

This is useful to run the servers through programs like valgrind for
debugging or eatmydata for performance.

chaozh/logcabin,TigerZhang/logcabin,logcabin/logcabin,logcabin/logcabin,dankenigsberg/logcabin,dankenigsberg/logcabin,chaozh/logcabin,TigerZhang/logcabin,gaoning777/logcabin,chaozh/logcabin,TigerZhang/logcabin,nhardt/logcabin,tempbottle/logcabin,gaoning777/logcabin,TigerZhang/logcabin,logcabin/logcabin,nhardt/logcabin,gaoning777/logcabin,nhardt/logcabin,tempbottle/logcabin,tempbottle/logcabin,tempbottle/logcabin,gaoning777/logcabin,dankenigsberg/logcabin,nhardt/logcabin,chaozh/logcabin,logcabin/logcabin,dankenigsberg/logcabin