test: Split database test case
test: Split database test case

The test_database_with_data_in_sstables_is_a_mutation_source case runs
the mutation source tests in one go. The problem is that on each step
a whole new ks:cf is created which takes the majority of the tests time.
In the end of the day this case is the slowest one in the suite being
up to two times longer (depending on mode) than the #2 on this list.

This patch splits the case into 4 so that each mutation source flavor
is run in separate case.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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