selftests: fix nested double quotes in f-string

Replace nested double quotes in f-string with outer single quotes.

Fixes: 6116075e ("selftests: nic_link_layer: Add link layer selftest for NIC driver")
Signed-off-by: David Wei <dw@davidwei.uk>
Link: https://patch.msgid.link/20241122064821.2821199-1-dw@davidwei.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>