Change tests to use async mode and add helpers and tests for schema changes.
These test series will be expanded with topology changes.
Closes#10550
* github.com:scylladb/scylla:
test.py topology: repro for issue #1207
test.py: port fixture fails_without_raft
test.py topology: table methods to add/remove index
test.py topology: add/drop table column helpers
test.py topology: insert sequential row
test.py: remove deprecated test test_null
test.py: managed random tables
test.py: test_keyspace fixture async
test.py: rename fixture test_keyspace to keyspace
test.py topology: test with asyncio