Tests using replica::table::add_sstable_and_update_cache() cannot
rely on the sstable being added to a single compaction group, if
the test was forced to run with multiple groups.
Additionally let's remove try_flush_memtable_to_sstable() which
is retricted to a single group, allowing the entire test to now
pass with multiple groups.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>