When two ogg streams were made with two different calls to `flac`
within the same second, their serial numbers would be the same.
This caused problems with seeking in the test suite. While in
theory, libFLAC should be able to seek in a chained stream with
two equal serial numbers without fail, in practice seeking in
chained streams is hard enough. So, this commit makes sure the
chained test files have links with unequal serial numbers.

Concerning the use of specific casts, shifts and masks: For some
reason, on some platforms, seemingly, saturating arithmetic was
used where overflow was expected, in effect always seeding with
the same number, UINT32_MAX.