sstable_mutation_test: strengthen tombstone_merging test
sstable_mutation_test: strengthen tombstone_merging test

In the tombstone_merging test, we expected one row tombstone. But we did
not verify that in addition to that row tombstone, there is no other rows
(deleted or otherwise). It turns out that in the onld merging algorithm,
we did produce additional deleted rows which shouldn't have been there.

So this patch adds a test that there are no such additional deleted rows
beyond the one row tombstone we expect. The test passes with the new
range tombstone merging algorithm.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <1459778074-10759-2-git-send-email-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

raphaelsc/scylla,duarten/scylla,kjniemi/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,kjniemi/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla,avikivity/scylla,raphaelsc/scylla,kjniemi/scylla,scylladb/scylla