Reduce the data size in test_checkpoint04 so it completes quickly

test_checkpoint04.test_checkpoint04.test_checkpoint_stats runs about 10 minutes.  In working on WT-6097, it was clear that the one bucket that had this test was much slower than all other buckets. It dramatically slows down PR testing. It can be tagged with:

 @wttest.longtest("filling all these tables is slow")