icalendar-tests no longer assumes TZ = wall

icalendar-tests no longer assumes TZ = wall

Do not assume that the TZ environment variable is either unset or
agrees with /etc/localtime.  I ran into this test bug while
running ‘TZ=America/Chicago make check’ on a platform where
/etc/localtime was America/Los_Angeles.
* test/lisp/calendar/icalendar-tests.el:
(icalendar-tests--decode-isodatetime): Don’t use
set-time-zone-rule, whose doc string says “Instead of calling this
function, you typically want something else” for a reason.
Instead, pass the zone arg to icalendar-test--format.