The big problem is that most tests in that file use some variant of _call as their entry point; they will need to change to calling more specific targets, which basically means rewriting a lot of tests.
One small cleanup this would allow: switching things like from letsencrypt.plugins import disco as plugins_disco ... plugins_disco.PluginsRegistry.find_all() to just from blah import PluginsRegistery .. PluginsRegistry.find_all().
I think this should definitely be done at some point. I think it will almost be easiest to write the tests from scratch.
If someone takes a crack at this, it'll be much easier to write them as unit tests, testing only a specific function or two at a time, instead of always calling main.main and having to mock out a dozen classes/functions.