Dep: (Also) Run test harness in-process

[sdboyer]

Right now, the test harness is executed out-of-process by compiling an instance of dep and Exec()ing it. It'd be lovely if we could _also_ make it work in-process, by mocking up the commands as needed and just calling them directly.

Doing this would provide us with proper code coverage information. While I'm not a coverage zealot, I do think it's very valuable for us to at least know which parts of our code are covered by tests, and which aren't.

If there's a way to get coverage information from the subprocess, though, I'm probably fine with that, too.

[carolynvs]

Another plus side of in-proc tests is easier debugging. When it spawns a separate process, I end up having to put in lots of temporary debug statements. 😀 Or maybe I just need to level up my debugger-fu.

[sdboyer]

Sure seems like another benefit to me 😄

/me confesses to not using a step-through debugger (in Go) 😝

[carolynvs]

I only use delve in anger. 😇

[jmank88]

I can work on this issue.
After exploring the code for a bit, I expect the largest part of the change will be organizing logging, since there are global function calls, global state, and direct os.Std* access sprinkled around which need to be captured by the tests. Some of these already have preexisting TODOs by virtue of being global, so that's another bonus for this issue.

[sdboyer]

@jmank88 aaaaawesome!! yes, your survey of the code matches with my sense of what we'd need to do - a bunch more encapsulation to get rid of the global state. That's some tech debt I would LOVE to pay down.

Once that's done, I suspect the rest oughtn't be that hard. Even better, it'll likely force us to maintain a higher level of quality/avoid global state in the future! 🎉