Port timestamp tests to odd timezones, (TICKS . HZ)

Port timestamp tests to odd timezones, (TICKS . HZ)

* test/src/timefns-tests.el:
(format-time-string-padding-minimal-deletes-unneeded-zeros)
(format-time-string-padding-minimal-retains-needed-zeros)
(format-time-string-padding-spaces)
(format-time-string-padding-zeros-adds-on-insignificant-side):
Don't assume local time can represent 2000-02-15 00:00:00,
as there might be a DST jump over midnight.
Work even when timestamps are of (TICKS . HZ) form.
Simplify by avoiding need to call time-add.