tests should only be run against the associated handler

[numberMumbler]

currently, handlers' tests are not isolated: every handler is run for each test. This causes problems (for example) when checking comment counts in a test result, because multiple handlers might add comments

[jdm]

We could probably support this by making get_handlers take an argument that is used to filter the resulting handlers that are discovered?

[numberMumbler]

I ended up adding it to the test setup, since both the handler and test were accessible at that point. Let me know if you'd like any changes