tests: improve capture of stdout/stderr
tests: improve capture of stdout/stderr

Use a single pipe for both, instead of a temporary file.

bowlofstew/scylla,guiquanz/scylla,kjniemi/scylla,aruanruan/scylla,glommer/scylla,duarten/scylla,raphaelsc/scylla,respu/scylla,guiquanz/scylla,bowlofstew/scylla,scylladb/scylla,bowlofstew/scylla,justintung/scylla,eklitzke/scylla,dwdm/scylla,glommer/scylla,capturePointer/scylla,asias/scylla,tempbottle/scylla,linearregression/scylla,raphaelsc/scylla,dwdm/scylla,acbellini/scylla,victorbriz/scylla,scylladb/scylla,kangkot/scylla,shaunstanislaus/scylla,justintung/scylla,justintung/scylla,rluta/scylla,stamhe/scylla,kangkot/scylla,phonkee/scylla,wildinto/scylla,phonkee/scylla,stamhe/scylla,asias/scylla,rluta/scylla,duarten/scylla,respu/scylla,duarten/scylla,gwicke/scylla,linearregression/scylla,victorbriz/scylla,avikivity/scylla,senseb/scylla,glommer/scylla,rentongzhang/scylla,senseb/scylla,raphaelsc/scylla,respu/scylla,linearregression/scylla,capturePointer/scylla,rentongzhang/scylla,rluta/scylla,victorbriz/scylla,eklitzke/scylla,kjniemi/scylla,kangkot/scylla,stamhe/scylla,scylladb/scylla,avikivity/scylla,tempbottle/scylla,wildinto/scylla,aruanruan/scylla,eklitzke/scylla,gwicke/scylla,rentongzhang/scylla,shaunstanislaus/scylla,tempbottle/scylla,scylladb/scylla,dwdm/scylla,wildinto/scylla,guiquanz/scylla,gwicke/scylla,kjniemi/scylla,asias/scylla,phonkee/scylla,aruanruan/scylla,acbellini/scylla,shaunstanislaus/scylla,acbellini/scylla,avikivity/scylla,capturePointer/scylla,senseb/scylla