test-parse-datetime.c: avoid new DST-related false positive test failure

test-parse-datetime.c: avoid new DST-related false positive test failure

* tests/test-parse-datetime.c (gmt_offset): Determine the "gmt_offset"
based on the time/date we'll convert, not the current time.
Otherwise, the moment we cross a DST boundary like today's in
Europe, (CEST to CET), that offset ends up being one hour off.