Fix test for pfcwd_sw_enable in db_migrator_test

[bingwang-ms]

What I did

This PR is to fix the input and output for testing the migration of pfcwd_sw_enable.
Before this change,

resulting_table = dbmgtr.configDB.get_table('PORT_QOS_MAP')
expected_table = expected_db.cfgdb.get_table('PORT_QOS_MAP')

resulting_table and expected_table are always empty {}. So the test case can always pass.

How I did it

Fix the input and output for testing the migration of pfcwd_sw_enable.

How to verify it

Run UT for db_migrator after update.

collected 75 items                                                                                                                                                                                    

tests/db_migrator_test.py ...........................................................................                                                                                           [100%]

===================================================================================== 75 passed in 17.14 seconds ======================================================================================

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)