tests-1.3: add bsn_flow_checksum testsuite

Reviewer: @harshsin

Testcases for https://github.com/floodlight/indigo/pull/95.

See the Loxigen extension bsn_flow_checksum. This extension maintains
checksums per table and for a configurable number of buckets in each table.
The checksums are sent to the switch in the cookie field of the flow-add.
These checksums are intended to be used for a hash-tree like synchronization
scheme.