Account for rounding errors in some places, and that two nows are
not always entirely identical, so allow some leeway when comparing
microseconds and seconds. Ran into this too often, esp. when the
system is under load.
Fix returning of timezones on systems with gdatetime
to use floats on the math expression to avoid
truncating the fractional part.
Also adds a test for covering this case.
Adds 2 variants for the gst_date_time_from_unix_epoch function,
one for UTC and another for local time.
API: gst_date_time_new_from_unix_epoch_utc
API: gst_date_time_new_from_unix_epoch_local_time
Fixes#653031https://bugzilla.gnome.org/show_bug.cgi?id=635031
