ring: simplify unit tests

ring: simplify unit tests

As this unit test does not create devices anymore,
and uses devices created by EAL option --vdev,
there were unnecesary tests that were repeated,
so they have been removed.

So now there are three tests:

1 - Test two devices that share a ring, one device
    with just one RX queue and the other with one
    TX queue.

2 - Test a device connected to itself (loopback) by
    a ring, with both RX and TX queue.

3 - Test two devices that share a ring, but both devices
    with RX and TX queue, so they can send packets to themselves
    and to the other device.

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>