test.py: move 'success' to TestUnit class
test.py: move 'success' to TestUnit class

There will be other success attributes: program return
status 0 doesn't mean the test is successful for all tests.

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