Fix bug in display

Previously, the plugin displayed:
```
aThe uptime did not change since the last check cycle for these node(s): , b
```

Change-Id: Ibfd4696d4152e86262ea653eb207273cee5fa040

cmk/base/plugins/agent_based/if64.py

@@ -131,7 +131,7 @@ def _check_timestamps(
         yield Result(
             state=State.OK,
             notice="The uptime did not change since the last check cycle for these node(s): "
-            ", ".join(nodes_without_uptime_increase)
+            + ", ".join(nodes_without_uptime_increase)
             + "\n"
             "It is likely that no new data was collected.",
         )
@@ -145,7 +145,7 @@ def _check_timestamps(
         yield Result(
             state=State.OK,
             notice="The uptime has decreased since the last check cycle for these node(s): "
-            ", ".join(nodes_without_uptime_increase)
+            + ", ".join(nodes_without_uptime_increase)
             + "\n"
             "The device might have rebooted or its uptime counter overflowed.",
         )

tests/unit/cmk/plugins/collection/agent_based/test_if64.py

@@ -0,0 +1,35 @@
+#!/usr/bin/env python3
+# Copyright (C) 2024 Checkmk GmbH - License: GNU General Public License v2
+# This file is part of Checkmk (https://checkmk.com). It is subject to the terms and
+# conditions defined in the file COPYING, which is part of this source code package.
+
+from cmk.base.plugins.agent_based import if64
+from cmk.base.plugins.agent_based.agent_based_api.v1 import Result, State
+
+
+def test_check_timestamps_decrease() -> None:
+    value_store: dict[str, object] = {}
+    assert not list(if64._check_timestamps({"a": 1, "b": 2}, value_store))  # pylint: disable=protected-access
+    assert list(if64._check_timestamps({"a": 0, "b": 1}, value_store)) == [  # pylint: disable=protected-access
+        Result(
+            state=State.OK,
+            notice="The uptime has decreased since the last check cycle for these node(s): \nThe device might have rebooted or its uptime counter overflowed.",
+        )
+    ]
+
+
+def test_check_timestamps_no_change() -> None:
+    value_store: dict[str, object] = {}
+    assert not list(if64._check_timestamps({"a": 1, "b": 2}, value_store))  # pylint: disable=protected-access
+    assert list(if64._check_timestamps({"a": 1, "b": 2}, value_store)) == [  # pylint: disable=protected-access
+        Result(
+            state=State.OK,
+            notice="The uptime did not change since the last check cycle for these node(s): a, b\nIt is likely that no new data was collected.",
+        )
+    ]
+
+
+def test_check_timestamps_valid() -> None:
+    value_store: dict[str, object] = {}
+    assert not list(if64._check_timestamps({"a": 1, "b": 2}, value_store))  # pylint: disable=protected-access
+    assert not list(if64._check_timestamps({"a": 61, "b": 62}, value_store))  # pylint: disable=protected-access