selftests: rtnetlink: fix addrlft test flakiness on power-saving systems

selftests: rtnetlink: fix addrlft test flakiness on power-saving systems

Jakub reported that the rtnetlink test for the preferred lifetime of an
address has become quite flaky. The issue started appearing around the 6.16
merge window in May, and the test fails with:

    FAIL: preferred_lft addresses remaining

The flakiness might be related to power-saving behavior, as address
expiration is handled by a "power-efficient" workqueue.

To address this, use slowwait to check more frequently whether the address
still exists. This reduces the likelihood of the system entering a low-power
state during the test, improving reliability.

Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20250715043459.110523-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>