sstables_manager superseeds previous implementation of sstables_tracker
for tracking lifetime of the tables. Update scylla-gdb.py to use
sstables_manager in a backwards compatible way, as sstables_manager is
not available in Scylla Enterprise 2020.1. Add explicit test for
"scylla sstables" command, as previously only "scylla active-sstables"
was tested.
Closes#9439