There's one in the utils that creates lw_shared_ptr<memtable> and
applies provided vector of mutations into it. Lots of other test cases
do literally the same by hand.
The make_memtable() assumes that the caller is sitting in the seastar
thread, and all the test cases that can benfit from it already are.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>