tests/test_rtsock_l3: fix flaky temporal route addition tests

If RTM_DELETE arrives before RTM_ADD, the test will skip the first
reply and then timeout after attempting to read another message from
the rtsock_fd.

See the CI test failure[0] for more details.

[0] https://ci.freebsd.org/job/FreeBSD-main-riscv64-test/16628/testReport/sys.net.routing/test_rtsock_l3/rtm_add_v6_temporal1_success/

Reviewed by: pouria
Fixes: e02d3fe70c7247027c85d60179c331618554ba34
MFC after: 3 days
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 9e2d8ec2df414a8019d9c31a9a09a5cc728b4c19)