test: add isolation level requirement message to tracing tests

Alternator tracing tests require the cluster to have the 'always'
isolation level configured to work properly. If that's not the case,
the tests will fail due to not having CAS-related traces present
in the logs. In order to help the users fix their configuration,
a helper message is printed before the test case is performed.
Automatic tests do not need this, because they are all ran with
matching isolation level, but this message could greatly improve
the user experience for manual tests.
Message-Id: <62bcbf60e674f57a55c9573852b6a28f99cbf408.1607949754.git.sarna@scylladb.com>

test/alternator/test_tracing.py

@@ -188,6 +188,8 @@ def test_tracing_all(with_tracing, test_table_s, dynamodb):
 
     # Check the traces. NOTE: the following checks are fairly arbitrary, and
     # may break in the future if we change the tracing messages...
+    print("If you see CAS failures here, please check if your alternator cluster is configured"
+            " to have write isolation level 'always'. Otherwise, CAS operations will not be present in the traces.")
     expect_tracing_events(dynamodb, p_putitem, ['PutItem', 'CAS successful'])
     expect_tracing_events(dynamodb, p_getitem, ['GetItem', 'Querying is done'])
     expect_tracing_events(dynamodb, p_deleteitem, ['DeleteItem', 'CAS successful'])