Add loadNetTest to NetTest
Add loadNetTest to NetTest

loadNetTest populates the test_cases from a file on disk or a file-like object
like StringIO
(cherry picked from commit c1cabc5a0e510cf38a59a5591630c61cb0983978)

Conflicts:
	ooni/measurements.py

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