sharder: trivial tidy up of unit tests
sharder: trivial tidy up of unit tests

Move one of the unit tests covering *root* container audit that was
located between tests covering *shard* containers and their helper
method.

Rename, add comments and add clarifying sanity checks to other unit
tests.

Change-Id: I2bffea2d123d8a345dac53159bc02c8b298153b8

openstack/swift,openstack/swift,openstack/swift,openstack/swift