test/alternator: test for change in CDC preimage
test/alternator: test for change in CDC preimage

In pull request #8568, the CDC API changed slightly, with preimage data
gaining extra "delete$k" values for columns whose preimage was missing.
In this new test, we verify that this change did not break Alternator.
We didn't expect it to break Alternator, because it just outputs the known
base-table columns and ignores the columns which weren't a real base-table
column - like this "delete$k".

In the test we set up a stream with preimages, ensure that a real column
(note that an LSI key is a real column instead of a map element) has a
null preimage - and see that the preimage is returned as expected,
without fake columns like "delete$k".

The test passes, showing that PR #8568 was ok.
The test also passes, as expected, on DynamoDB.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Message-Id: <20210504120121.915829-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

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