Add some black-box testing

For the first time, perform black box testing by (indirectly) calling
'cmd.main'.

The fixtures implemented here take of things like patching argv and
setting up a bucket to act as a target for the tests.

Besides improving coverage, the idea is that adding analogous
components to the class `AwsTestConfig` and then modifying
`_make_fixture_param_and_ids` will allow for easier, portable smoke
testing of more storage backend functionality.

@boldfield and @alex may be interested, because part of the draw is
getting more testing of the other storage back-ends for relatively low
cost, as well as getting in a position to improve coverage in general.