First I wanted to fix

  test:ERROR:../src/ndisc/tests/test-ndisc-fake.c:373:test_preference_changed_cb: assertion failed (_a->timestamp == (data->timestamp1 + 3)): (9 == 10)

but that leads to a different failure:

  test:ERROR:../src/ndisc/tests/test-ndisc-fake.c:375:test_preference_changed_cb: assertion failed (_a->lifetime == (9)): (10 == 9)

Instead, the start and end times must match exact (in their duration),
we only allow them to be shifted by up to one second.

Fixes: 8209095e ('ndisc/tests: relax the assertion in "test-ndisc-fake.c"')