possible approach for "test tags"

[Eric Snow]

We've had discussions here a couple of times (since I joined the team)
about classifying tests in our suite so they could be run more
flexibly.  We've also already added explicit handling for a specific
kind of test along those same lines: featuretests.  Additionally you
can think of the CI tests as another test classification.  So we're
already taking the approach in a limited fashion.

I'd like to see something more generic that we could use in the juju
test suite.  I've put up a proof of concept patch that demonstrates
what I think would fit the bill:

  http://reviews.vapour.ws/r/1647/

If the approach seems reasonable then we could start using it for new
tests and as appropriate when touching existing code.

-eric