utils: add unittest coverage

We had some Python 2/3 issues related to command output, but we
didn't have any test coverage to catch it.  Add better unittest
coverage for this module and fix up some issues it uncovers.

Bug: 28298984
Test: unittests pass for py2 & py3
Change-Id: I13300428b4667393b29af466a795effd6374f392