When destroying an perf_sstable_test_env, an assert in sstables_manager
destructor fails, because it hasn't been closed.
Fix by removing all references to sstables from perf_sstable_test_env,
and then closing the test_env(as well as the sstables_manager)
Fixes#8736
Signed-off-by: Wojciech Mitros <wojciech.mitros@scylladb.com>
Closes#8737