test: schema_changes_test: properly close sstables::test_env
test: schema_changes_test: properly close sstables::test_env

sstables::test_env needs to be properly closed (and will soon need it
even more). Use test_env::do_with_async() to do that. Removed
await_background_jobs(), which is now done by test_env::close().

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