test: fix ring PMD initialisation

test: fix ring PMD initialisation

(bitratestats_autotest|latencystats_autotest|pdump_autotest) tests
generate a log of error messages like that:

test_packet_forward() line 104: Error sending packet to port 0
Send pkts Failed

These tests use of app/test/sample_packet_forward.* code.
This code creates a portid from a ring, but doesn't properly
configure/start it.
The fix adds code to configure/start given port before usage.

Fixes: 7a0935239b9e ("ethdev: make fast-path functions to use new flat array")
Fixes: a52966cd48fd ("test: add helpers using ring PMD Rx/Tx")
Cc: stable@dpdk.org
Reported-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Tested-by: David Marchand <david.marchand@redhat.com>