[Backport 6.2] fix(test_user_batch_custom_time): handle stress command correctly

[mergify]

seems like we were running into situations we try to add all nodes ips into stress commands, and it's the test code

File ".../longevity_test.py", line 300, in test_user_batch_custom_time
self._run_user_stress_in_batches(batch_size=batch_size,
File ".../longevity_test.py", line 336, in _run_user_stress_in_batches
batch_params['stress_cmd'] += [params['stress_cmd'] + nodes_ips]
TypeError: list indices must be integers or slices, not str

in this change remove a parentheses that was causing function to return tuple and not string as expected

Testing

• 🟢 https://argus.scylladb.com/tests/scylla-cluster-tests/8d280858-295a-482a-970e-e62bfcfed597

PR pre-checks (self review)

• I added the relevant backport labels
• I didn't leave commented-out/debugging code

Reminders

• Add New configuration option and document them (in sdcm/sct_config.py)
• Add unit tests to cover my changes (under unit-test/ folder)
• Update the Readme/doc folder relevant to this change (if needed)

(cherry picked from commit 8f16d05)

Parent PR: #9345