test: vtgate_buffer.py: Stop the the two threads in case of a test error.
test: vtgate_buffer.py: Stop the the two threads in case of a test error.

Without this change, an error would be logged but the test would keep running (forever?).
sougou/vitess,alainjobart/vitess,vitessio/vitess,enisoc/vitess,applift/vitess,dcadevil/vitess,HubSpot/vitess,rnavarro/vitess,applift/vitess,tirsen/vitess,tirsen/vitess,rnavarro/vitess,mattharden/vitess,rnavarro/vitess,mahak/vitess,rnavarro/vitess,vitessio/vitess,tirsen/vitess,mattharden/vitess,dcadevil/vitess,mahak/vitess,tinyspeck/vitess,pivanof/vitess,HubSpot/vitess,dcadevil/vitess,pivanof/vitess,tinyspeck/vitess,vitessio/vitess,HubSpot/vitess,tirsen/vitess,rnavarro/vitess,rnavarro/vitess,vitessio/vitess,HubSpot/vitess,pivanof/vitess,pivanof/vitess,rnavarro/vitess,tirsen/vitess,sougou/vitess,tirsen/vitess,tinyspeck/vitess,pivanof/vitess,tinyspeck/vitess,mattharden/vitess,sougou/vitess,sougou/vitess,enisoc/vitess,sougou/vitess,alainjobart/vitess,enisoc/vitess,sougou/vitess,davygeek/vitess,tirsen/vitess,tinyspeck/vitess,enisoc/vitess,tinyspeck/vitess,mattharden/vitess,mahak/vitess,applift/vitess,vitessio/vitess,enisoc/vitess,mahak/vitess,mattharden/vitess,tinyspeck/vitess,mattharden/vitess,mahak/vitess,sougou/vitess,alainjobart/vitess,alainjobart/vitess,applift/vitess,mattharden/vitess,applift/vitess,davygeek/vitess,HubSpot/vitess,mahak/vitess,dcadevil/vitess,tirsen/vitess,davygeek/vitess,applift/vitess,enisoc/vitess,mahak/vitess,pivanof/vitess,applift/vitess,vitessio/vitess,alainjobart/vitess,alainjobart/vitess,HubSpot/vitess,davygeek/vitess,dcadevil/vitess,davygeek/vitess,mattharden/vitess,applift/vitess,mattharden/vitess,pivanof/vitess,mahak/vitess,dcadevil/vitess,pivanof/vitess,rnavarro/vitess,alainjobart/vitess,davygeek/vitess,dcadevil/vitess,mattharden/vitess,davygeek/vitess,alainjobart/vitess,applift/vitess,pivanof/vitess,sougou/vitess,HubSpot/vitess,enisoc/vitess,vitessio/vitess,tirsen/vitess,applift/vitess,rnavarro/vitess,vitessio/vitess,HubSpot/vitess