Add TestParser

The TestParser class knows how to parse the output produced by the
ooniprobe tests. The idea is to seperate the parsing logic from the Test
class.
Add parsable output to tests

Ihis makes it easy for TestParser to keep track of the test results,
without having to parse the .yamloo reports.
Add test subclasses

One small subclass per test, to simplify the process of creating and running
tests.
Rewrite the TestCase class