Add imports and fix referenced-before-assignment

Adds required imports and fixes a bug where no test-helper is
requested but requested_helper is referenced without being assigned.
Update data/policy.yaml to use the NetTest name.

Some of the NetTest name strings do not match the convention used
by ooni-probe. Note that we normalize the NetTestClass.name field.
See : ooni-probe/ooni/nettest.py:195 and 392