Fix decode-time/encode-time roundtrip on macOS

* src/timefns.c (Fencode_time): Ignore DST flag when the zone is
numeric or is a cons, as the doc string says it’s ignored in that
case, and not ignoring it causes encode-time to not invert
decode-time on some platforms (Bug#35502).
* test/src/timefns-tests.el (encode-time-dst-numeric-zone):
New test.
1 job for master in 64 minutes and 38 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
passed #1593
test-all

01:04:38