Test suite: really test that we don't leak "amnesia" in DHCP requests (refs: #16365)

On Tails 3.11, hostname(1) returns… "localhost.localdomain".
I assume that's been the case since #13574 and #15322.
So we were not testing whether "amnesia" leaks in DHCP requests anymore.

Let's check for both "amnesia" and whatever hostname(1) returns.