alternator-test: enable basic read-before-write cases
alternator-test: enable basic read-before-write cases

With unsafe read-before-write implemented, simple cases can be enabled
by removing their xfail flag.

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