Rewrites trickiest cmd tests to help clarify what's happening

[codefromthecrypt]

Before, the cmd unit tests were difficult to understand. They are still
complicated and difficult, but far better documented now. Moreover, by
using standard testing libs, individual tests and even table cells can
be driven by the IDE, and failures point to a relevant location in a
table also.

See #130 for further rational about ginkgo/gomega vs testing/testify

[codefromthecrypt]

opened #154 about the flakey test as there are about 4 that could break the same way

[mathetake]

Finally I've looked at every case and understood them. LGTM so far (I want to have cmd_test.go free of Ginkgo as in my comment).

[codefromthecrypt]

thanks for the hours spent on an epic review @mathetake

It took me several days to understand the tests to write this, so I'm glad two of us understand now. Most importantly, once this is in, we can fix some really concerning root problems that make tests so difficult, such as reliance on actually invoking executables vs stubbing. For now, I'm glad we can progress to make future progress less trouble. Will address your commits now.

[codefromthecrypt]

actually there are still a lot of tests left in the package. To save you from a blinding amount of unrelated changes, I kept the code related to your direct comments and commit to migrating the other tests in a subsequent PR, sg @mathetake?

[mathetake]

sg

[codefromthecrypt]

re-kicked as the osx thing is busted

[codefromthecrypt]

appreciate your at least 4 hours on this @mathetake!