A fixed offset (e.g. the "UTC" keyword) was applied after relative
items, so "1970/01/01 UTC N seconds" added N seconds to a local
wall-clock that could drift across a DST boundary before being
re-anchored at the offset, yielding an instant off by the DST gap.
Anchor the offset first, then apply relative items.
Fixesuutils/coreutils#12555.
NOTE: This commit only adds tests as the implementation was already
working correctly. These tests document and verify the expected
behavior for the embedded timezone edge case referenced in Debian
bug #851934.
Sylvestre Ledru
Chris Dryden
yuankunzhang
Daniel Hofstetter
Maxim