Add unit tests for the combined reader - selector interactions
Add unit tests for the combined reader - selector interactions

There are a few edge cases that were untested and as this patch-series
reworks completely how the combined-reader works these should be tested
as well to ensure they keep working.

scylladb/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla