Adapt the tests to not care about failures from platform gmtime()

Adapt the tests to not care about failures from platform gmtime()

Now that we don't use gmtime, timegm and friends from the platform
we should be able to handle all values the same on any platform
with no concerns other than what the platform has decided to use
for time_t.

The hpux #ifdef is removed, as looking at f5e3115361f shows this
was added due to a gmtime() failure on the platform.

This then clears out all the platform specific chaos in helper.c
as an example of why the public API being added is actually
generally useful, as it has been noticed that users of OpenSSL
frequently end up dealing with the same issues in the same really
awful ways in their own software as were were doing in that file
in the tests.

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28748)