Since `flat_reader_assertions::produces(const range_tombstone&,...)`
records the range tombstone for checking, be sure to explicitly pass
in a clustering range that does not extend beyond the mock-read part
of the mutation.
Also (provisionally) change the assertion method to accept clustering
ranges.
Signed-off-by: Michael Livshin <michael.livshin@scylladb.com>
2221aeff0e