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

* test/src/timefns-tests.el:
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.
1 job for master in 52 minutes and 28 seconds (queued for 1 second)
Status Job ID Name Coverage
passed #5106