pack() uses a coarse-grain timer to decide which revisions to look at
during a pack.  The tests depend on a pack() called after a commit to
see that committed transaction, but the implementation can't guarantee
that it will.

In particular, pack() converts its time argument to a TimeStamp by
truncating to the nearest second.  If a transaction T1 commits and pack
is called (with the current time as its argument) during the same second,
the pack() will not consider T1.  This was causing intermittent, but
frequent test failures on Windows.

The fix is to add a small delay in the pack test cases to guarantee that
pack(time.time()) will always see the last committed transaction.