test: sstable_3_x_test: remove test_sstables_manager references
test: sstable_3_x_test: remove test_sstables_manager references

test_sstables_manager is going away, so replace it by test_env::manager().

column_family_test_config() has an implicit reference to test_sstables_manager,
so pass test_env::manager() as a parameter.

Calls to await_background_jobs() are removed, since test_env::stop() performs
the same task.

The large rows tests are special, since they use a custom sstables_manager,
so instead of using a test_env, they just close their local sstables_manager.

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