Change test harness to support testing full output of a command?

[carolynvs]

At the moment it would be easy to accidentally break verbose logging (#968), and not realize until someone opens a bug. It would be useful to have a harness test for each command that checks the full command output (not just stdout, since verbose is sent to stderr).

Tests

• dep init -v
• dep ensure -vendor-only -v
• dep ensure -vendor-only -dry-run
• dep prune -v
• dep prune -v -dry-run
• ... there's more in this matrix I'm sure

The harness currently supports checking stdout, and separately stderr, which is awkward for this type of test, where the output is interleaved. If we could have another file in the testcase directory, like output.txt, which was included in the -update golden flag, that would be easier.

[sdboyer]

yeah. i was just thinking about the need for this while reviewing #968. we should definitely have this, at minimum so that we can make an explicit choice to accept when outputs are changing.