Improve test cases in the tsdb package for the ShardGroup interface
Improve test cases in the tsdb package for the ShardGroup interface

tsdb.Shard and tsdb.Shards both implement tsdb.ShardGroup and neither
were tested within the tsdb package itself. This adds tests for those
methods which are used by the query engine.

influxdata/influxdb,influxdata/influxdb,nooproblem/influxdb,benbjohnson/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,nooproblem/influxdb,li-ang/influxdb,influxdb/influxdb,benbjohnson/influxdb,chenziliang/influxdb,benbjohnson/influxdb,vladlopes/influxdb,li-ang/influxdb,influxdb/influxdb,benbjohnson/influxdb,linearb/influxdb,vladlopes/influxdb,linearb/influxdb,vladlopes/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,influxdb/influxdb,chenziliang/influxdb,linearb/influxdb,mark-rushakoff/influxdb,vladlopes/influxdb,mark-rushakoff/influxdb,influxdb/influxdb,chenziliang/influxdb,chenziliang/influxdb,mark-rushakoff/influxdb,linearb/influxdb,linearb/influxdb,influxdb/influxdb