test: load drivers when required

Concatenating to test_args if event_eth_tx_adapter_autotest is executed
makes all subsequent tests inherit from the drivers loading while this
is unneeded.

Fixes: 207b1c813f39 ("test: fix build without ring PMD")
Cc: stable@dpdk.org
Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Aaron Conole <aconole@redhat.com>