Removes the "testing" package import automatically

It shouldn't be necessary if you're using Ginkgo. We do need a good
replacement for stretchr/testify for some projects, but that's an
entirely different problem.
Adds --destructive flag

Eventually I'm going to be happy with how this works and will to want to
convert a bunch of tests at work. Rapidly approaching that point, but it
would be nice to finish these features and start using this tool.