Add a Go test for LAG LACP partner against OTG back-to-back topology

[ashutshkumr]

• The test should configure LAG (and LACP) with one member port (min link = 1)
• The test should configure a unidirectional UDP flow with LAG ports as endpoints
• The test should fetch LACP metrics to ensure partner system ID is as expected
• The test should fetch LAG metrics to ensure oper status is up and tx/rx frames are as expected