rt-tests: fix irq_test_priority to retrieve only the interface irq thread
rt-tests: fix irq_test_priority to retrieve only the interface irq thread

The test uses ps to try and find the irq used by the network device, but
in the case where the irq is shared, you can end up with a space-separated
list of results; e.g. by searching only for "irq/$dev_irq", we get "71 72
391 3613" out of:

      71 irq/10-uhci_hcd
      72 irq/10-uchi_hcd
     391 irq/10-virtio0
    3613 irq/10-eth0

Tighten the grep pattern to include the interface name; this narrows us
down to a single result, making the test work as expected.

Natinst-AzDO-ID: 1050836
Signed-off-by: Brandon Streiff <13fa32f2726d04b7021cfff7b1046af0e9dba2df@ni.com>

ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt,ni/meta-nilrt