Add a "SKIP" status

[AdamIsrael]

I've run into cases where tests are known to fail in certain circumstances but shouldn't be counted against the test run on the whole. For example:

• A charm that requires customer-specific authentication (by env variable, config, etc.) to enable functionality.
• A charm is known not to work on certain providers, i.e., local/lxd

So, in addition to PASS and FAIL I propose we also enable a test to emit SKIP in events where a test cannot or should not run in order for automated tests to accurately reflect the tested state of the charm (like cloud-weather-report).

[johnsca]

There's some overlap with juju-solutions/build-cloud#13