fix: slow regression tests tests

[kostasrim]

We have a few very slow tests on the CI. Specifically:

241.30s call     dragonfly/replication_test.py::test_replication_all[df_factory0-mode0-8-t_replicas8-seeder_config8-50000-True]
240.80s call     dragonfly/replication_test.py::test_replication_all[df_factory0-mode0-8-t_replicas7-seeder_config7-50000-False]
239.92s call     dragonfly/replication_test.py::test_replication_all[df_factory0-mode1-8-t_replicas7-seeder_config7-50000-False]
236.87s call     dragonfly/replication_test.py::test_replication_all[df_factory0-mode1-8-t_replicas8-seeder_config8-50000-True]
178.18s call     dragonfly/snapshot_test.py::test_big_value_serialization_memory_limit[HSET-df_factory0]
175.97s call     dragonfly/snapshot_test.py::test_big_value_serialization_memory_limit[SADD-df_factory0]
175.79s call     dragonfly/snapshot_test.py::test_big_value_serialization_memory_limit[ZSET-df_factory0]
172.35s call     dragonfly/snapshot_test.py::test_big_value_serialization_memory_limit[LIST-df_factory0]
60.21s call     dragonfly/connection_test.py::test_pubsub_busy_connections[df_factory0]

This PR changes:

It refactors the test_big_value_serialization. First, the test was incorrect, in fact for some reason it did not even run with big value serialization flag set. Second, the test used execute_command in a loop which took a substantial amount of time. The changes on my machine reduce the total running time of the test from 4 * 170 seconds on average to a staggering 1 minute and 10 seconds for all 4.

Also notice, that we don't really need to test big value serialization on a stress test. It's redundant because when we set the value to a small number (4096 etc) it will have the same effect regardless of the stress load. I removed that one test case, saving as roughly (235 * 2) seconds

• refactor test_big_value_serialization
• remove no needed replication tests for big value

[kostasrim]

Impact on a full run: https://github.com/dragonflydb/dragonfly/actions/runs/11796502461

(I will post the results once it completes)

[kostasrim]

x86 debug 28 minutes now takes 22 arm debug 42 minutes now takes 34 x86 release 28 minutes now takes 19 arm release 46 minutes now takes 28

Savings on a full run 24minutes (because arm tests run sequentially)

https://github.com/dragonflydb/dragonfly/actions/runs/11797723885/job/32862315798 vs https://github.com/dragonflydb/dragonfly/actions/runs/11793906726/job/32850430451

[kostasrim]

At some point I will take care of:

180.20s call dragonfly/replication_test.py::test_replicaof_reject_on_load[df_seeder_factory0-df_factory0]

It's the next low hanging fruit