cmd/stress: add "-ignore regexp" flag
cmd/stress: add "-ignore regexp" flag

This adds a flag for specifying a regular expression for failures that
should be ignored. This is useful for filtering out known issues and
provides a logical mirror to the existing -failure flag.

Change-Id: Ibbacdd2125aa23fe819896e5a17664b703c4ee35
Reviewed-on: https://go-review.googlesource.com/12676
Reviewed-by: Dmitry Vyukov <05e8181be3150e47da8fe2e1f18d381c75496ff4@google.com>

springning/tools,golang/tools,springning/tools,springning/tools,syreclabs/go-tools,golang/tools,syreclabs/go-tools,magiccao/tools,argandas/tools,magiccao/tools,cw2018/tools,runcom/tools,nicot/tools,cmars/tools,nicot/tools,argandas/tools,argandas/tools,jdhenke/tools,golang/tools,josharian/tools,golang/tools,golang/tools,josharian/tools,syreclabs/go-tools,josharian/tools,matryer/tools,syreclabs/go-tools,glasser/golang-tools,cmars/tools,cw2018/tools,syreclabs/go-tools,matryer/tools,cw2018/tools,springning/tools,jdhenke/tools,glasser/golang-tools,runcom/tools,matryer/tools,syreclabs/go-tools,glasser/golang-tools,nicot/tools,cmars/tools,nicot/tools,matryer/tools,argandas/tools,runcom/tools,runcom/tools,josharian/tools,magiccao/tools,cw2018/tools,magiccao/tools,glasser/golang-tools,jdhenke/tools,cmars/tools,jdhenke/tools