app/test: fix ring unit test

app/test: fix ring unit test

If eth_ring vdevs are created from the command line with the --vdev option,
they create their own rings which are not shared by other vdevs.
Some of tests in this suite require that the vdevs share rings, so some
of the tests fail.

For vdevs to share rings they must be created in the test code with the
rte_eth_from_rings() function using rings created with the rte_ring_create()
function.

Use the command line option --vdev=eth_ring0 to create port 0.
This option is not mandatory for the tests to pass. It allows some additional
functional tests on cmd-line created rings to be executed.

Create two rings and five ethdevs in test_pmd_ring.c for ports 1 to 5,
and then use these to run the unit tests on the pmd ring functionality.

Improve test output by adding the port number to printf statements,
and adding a printf describing each test.

Fixes: 572eb3cd834c ("ring: simplify unit tests")

Signed-off-by: Bernard Iremonger <bernard.iremonger@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>