Add support for having multiple test classes inside of a nettest file
This issue was automatically migrated from github issue https://github.com/TheTorProject/ooni-probe/issues/289.
Currently it is not possible to have more than one NetTestCase inside of a nettest file.
This means that something like the following will not work properly:
class TestA(nettest.NetTestCase): def test_foo(self): pass class TestB(nettest.NetTestCase): def test_bar(self): pass