alternator-test: add testing different write isolation policies
alternator-test: add testing different write isolation policies

Additional testing is done via:
1. Checking that permissive isolation levels ('a', 'o', 'u') allow
   conditional writes
2. Checking that 'f' isolation level (forbid rmw) works as expected:
   - read-modify-write requests are forbidden
   - non-rmw writes are allowed

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