Don't look up helpers if we have already specified a helper.
Don't look up helpers if we have already specified a helper.

Note: we will need to remove the default helpers in ooni's NetTests.

Karthikeyan-kkk/ooni-probe,Karthikeyan-kkk/ooni-probe,kdmurray91/ooni-probe,juga0/ooni-probe,kdmurray91/ooni-probe,lordappsec/ooni-probe,Karthikeyan-kkk/ooni-probe,lordappsec/ooni-probe,Karthikeyan-kkk/ooni-probe,0xPoly/ooni-probe,lordappsec/ooni-probe,juga0/ooni-probe,kdmurray91/ooni-probe,lordappsec/ooni-probe,0xPoly/ooni-probe,kdmurray91/ooni-probe,0xPoly/ooni-probe,juga0/ooni-probe,0xPoly/ooni-probe,juga0/ooni-probe