Add resume support for tests
It appears that the resume support isn't thread safe - if one has multiple tests running, we don't store the state or an identifier of the test in the resume file (ooniprobe.resume). This means that any number of tests running from a deck will be impossible to resume - we should probably hash the arguments to make a unique but easy to collide with identifier - then we can store the state of each unique call, even for multiple runs of the same test.