test, compaction: Do not rely on random timestamp
test, compaction: Do not rely on random timestamp

Again, there's a sub-case with sequential time stamps that still
works by chance. This time it's because splitting 256 sstables
into buckets of maximum 8 ones is allowed to have the 1st and the
last ones with less than 8 items in it, e.g. 3, 8, ..., 8, 5. The
exact generation depends on the time-since-epoch at which it
starts.

When all the cases are run altogether this time luckily happens
to be well-aligned with 8-hours and the generated buckets are
filled perfectly. When this particular test-case is run all alone
(e.g. by --run_test or --parallel-cases) then the starting time
becomes different and it gets less than 4 sstables in its first
bucket.

The fix is in adjusting the starting time to be aligned with the
8 hours window.

Actually, the 8 hours appeared in the previous patch, before which
it was 24 hours. Nonetheless, the above reasoning applies to any
size of the time window that's less than 256, so it's still an
independent fix.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla