groups-1.3: add tests for SELECT group forwarding

Tests empty, single-buckets, and multiple-bucket groups.

Does not test port-down or unequal-weight scenarios. Doesn't use any bucket
actions other than a single output.
Merge into master from pull request #76:
groups-1.3: add tests for SELECT group forwarding (https://github.com/floodlight/oftest/pull/76)