app/test: fix pci registration

app/test: fix pci registration

Since commit a155d430119 ("support link bonding device initialization"),
rte_eal_pci_probe() is called in rte_eal_init().
pci_autotest called it to bind devices to the test_driver and test_driver2.
Therefore, the function is called twice and devices already allocated
will cause the test fail.

This patch solves that issue, unregistering all previous drivers before
calling rte_eal_pci_probe() for the first time, so DPDK does not try
to allocate data for the devices, binding them to their previous
drivers again.

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>