kunit_tool: refactored kunit_tool for adding subcommands

Cleaned up kunit_tool a bit to make adding subcommands easier. Also
added some tests.

Google-Bug-Id: 117125357
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Change-Id: Ic032bb64c33f6f9a70b51a191caa7019eecdff02
2 files changed