test/alternator: make "run" script use only_rmw_uses_lwt

Originally (since commit c3da9f2), Alternator's functional test suite
(test/alternator) ran "always_use_lwt" write isolation mode. The original
thinking was that we need to exercise this more difficult mode and it's
the most important mode. This mode was originally chosen in
test/alternator/run.

However, starting with commit 76a766c (a year ago), test.py no longer
runs test/alternator/run. Instead, it runs Scylla itself, and the options
for running Scylla appear in test/alternator/suite.yaml, and accidentally
the write isolation mode only_rmw_uses_lwt was chosen there.

The purpose of this patch is to reconcile this difference and use the
same mode in test.py (which CI is using) and test/alternator/run (which
is only used by some developers, during development).

I decided to have this patch change test/alternator/run to use
only_rmw_uses_lwt. As noted above, this is anyway how all Alternator
tests have been running in CI in the past year (through test.py).
Also, the mode only_rmw_uses_lwt makes running the Alternator test
suite slightly faster (52 seconds instead of 58 seconds, on my laptop)
which is always nice for developers.

This patch changes nothing for testing in CI - only manual runs through
test/alternator/run are affected.

Signed-off-by: Nadav Har'El <nyh@scylladb.com>

test/alternator/run

@@ -65,7 +65,7 @@ def run_alternator_cmd(pid, dir):
     cmd += [
         '--alternator-address', ip,
         '--alternator-enforce-authorization', '1',
-        '--alternator-write-isolation', 'always_use_lwt',
+        '--alternator-write-isolation', 'only_rmw_uses_lwt',
         '--alternator-streams-time-window-s', '0',
         '--alternator-timeout-in-ms', '30000',
         '--alternator-ttl-period-in-seconds', '0.5',