before this change, memtable serves as the fixture for 6 test cases,
actually these 6 test cases can be categorized into a matrix of 3 x 2:
{ single_row, multi_row, large_partition } x { single_partition, multi_paritition }.
in this change, we break memtable into 3 different fixtures, to reflect
this fact. more readable this way. and a benefit is that each test does
not have to pay for the overhead of setup it does not use at all.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closesscylladb/scylladb#20177
ecfe0aace6