De-duplicate unit tests for ports in Big Switch

This eliminates the separate classes that existed
for the VIF type testing. Each class was going
through all of the port tests when none of the
logic those tests were exercising was affected by
the VIF types.

This cut the number of tests run in test_restproxy_plugin
by close to 44% without a reduction in code coverage.

Closes-Bug: #1295448
Change-Id: I8c84e1599568fad36b6dd3d51e4ba8a3c012c6c5
(cherry picked from commit 913f45bc16dd2c35f7be772d5867f9208064498a)
Merge "De-duplicate unit tests for ports in Big Switch" into milestone-proposed