test-time-util: suppress timestamp conversion failures for Africa/Khartoum timezone

test-time-util: suppress timestamp conversion failures for Africa/Khartoum timezone

Our timestamp conversion roundtrip test was failing. But I think that this
is not our bug:

$ TZ='Africa/Khartoum' date --date='@1509482094'
Tue Oct 31 23:34:54 EAT 2017
$ TZ='Africa/Khartoum' date --date='Tue Oct 31 23:34:54 EAT 2017' +%s
1509485694
$ TZ='Africa/Khartoum' date --date='@1509485694'
Tue Oct 31 23:34:54 CAT 2017
$ echo $[1509485694 - 1509482094]
3600

This is essentially the same as what happens in our test. After a round-trip, we
end up one hour ahead.

> For 1509482094632752, from the change log of tzdata:
>
> Release 2017c - 2017-10-20 14:49:34 -0700
>
>  Changes to future timestamps
>    Sudan will switch from +03 to +02 on 2017-11-01.

Fixes https://github.com/systemd/systemd/issues/28472.