test: utils.py: Set -stderrthreshold for vtgate.
test: utils.py: Set -stderrthreshold for vtgate.

After this change, test runs with -v will show the vtgate Go logs.
vitessio/vitess,applift/vitess,enisoc/vitess,mattharden/vitess,HubSpot/vitess,enisoc/vitess,HubSpot/vitess,vitessio/vitess,mattharden/vitess,pivanof/vitess,dcadevil/vitess,HubSpot/vitess,sougou/vitess,applift/vitess,dcadevil/vitess,tirsen/vitess,alainjobart/vitess,dcadevil/vitess,enisoc/vitess,applift/vitess,alainjobart/vitess,rnavarro/vitess,dcadevil/vitess,HubSpot/vitess,applift/vitess,tirsen/vitess,davygeek/vitess,mahak/vitess,alainjobart/vitess,vitessio/vitess,applift/vitess,mattharden/vitess,enisoc/vitess,tirsen/vitess,tirsen/vitess,vitessio/vitess,tirsen/vitess,rnavarro/vitess,vitessio/vitess,mahak/vitess,davygeek/vitess,vitessio/vitess,alainjobart/vitess,dcadevil/vitess,mahak/vitess,tinyspeck/vitess,davygeek/vitess,HubSpot/vitess,pivanof/vitess,rnavarro/vitess,sougou/vitess,mahak/vitess,alainjobart/vitess,tirsen/vitess,sougou/vitess,tinyspeck/vitess,alainjobart/vitess,sougou/vitess,pivanof/vitess,enisoc/vitess,davygeek/vitess,rnavarro/vitess,tirsen/vitess,mahak/vitess,mattharden/vitess,alainjobart/vitess,tinyspeck/vitess,sougou/vitess,rnavarro/vitess,rnavarro/vitess,davygeek/vitess,davygeek/vitess,mattharden/vitess,rnavarro/vitess,mattharden/vitess,tirsen/vitess,davygeek/vitess,pivanof/vitess,mattharden/vitess,vitessio/vitess,rnavarro/vitess,applift/vitess,mahak/vitess,HubSpot/vitess,applift/vitess,pivanof/vitess,dcadevil/vitess,sougou/vitess,mattharden/vitess,enisoc/vitess,tirsen/vitess,tinyspeck/vitess,vitessio/vitess,applift/vitess,HubSpot/vitess,tinyspeck/vitess,rnavarro/vitess,enisoc/vitess,mattharden/vitess,mahak/vitess,pivanof/vitess,dcadevil/vitess,HubSpot/vitess,tinyspeck/vitess,sougou/vitess,applift/vitess,pivanof/vitess,pivanof/vitess,applift/vitess,mattharden/vitess,pivanof/vitess,alainjobart/vitess,mahak/vitess,sougou/vitess,tinyspeck/vitess