Write some unittests for NetTest class
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
Move NetTest over to nettest module
Delete measurements module
Fix import inside of nettest
Rename test_manager to test_nettest

Remove duplicated tests
Fix imports in nettest unittest
Check to see if the test methods are callables
Minor cleaning of Measurement Task