tests/acceptance/migration: Add the 'migration' tag

By using an Avocado tag, we can run all tests described by that
tag as once:

  $ avocado --show=app run -t migration tests/acceptance/
  JOB ID     : 165477737509503fcfa6d7108057a0a18f2a6559
  JOB LOG    : avocado/job-results/job-2020-02-04T17.29-1654777/job.log
   (1/3) tests/acceptance/migration.py:Migration.test_migration_with_tcp_localhost: PASS (0.38 s)
   (2/3) tests/acceptance/migration.py:Migration.test_migration_with_unix: PASS (0.33 s)
   (3/3) tests/acceptance/migration.py:Migration.test_migration_with_exec: PASS (0.07 s)
  RESULTS    : PASS 3 | ERROR 0 | FAIL 0 | SKIP 0 | WARN 0 | INTERRUPT 0 | CANCEL 0

Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20200204163304.14616-1-philmd@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>