test: sstable_resharding_test: prepare for asynchronously closed sstables_manager
test: sstable_resharding_test: prepare for asynchronously closed sstables_manager

Close an explicity-created test_env, and stop using test_sstables_manager
which will disappear.

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