testutils: add delete_all_groups
testutils: update flow_msg_create for OF 1.1+
testutils: add verify_no_errors

This is useful when sending messages like flow-mods or group-mods that do not
have a reply. It should be called after a barrier to ensure the switch has
processed the previous requests.
groups-1.3: group table management tests

These tests were originally written by Rodrigo Denicol <rdenicol@cpqd.com.br>.
I updated them to the current OFTest API and made some other improvements.

These tests only cover add/modify/delete of groups, and the stats requests. No
packets are sent through the dataplane.
testutils: improve get_stats OF 1.3 support
groups-1.3: add boundary tests for group IDs
Merge into master from pull request #75:
group table management tests (https://github.com/floodlight/oftest/pull/75)