fix: do not claim update reading or applying QTS cannot do

QTS has no apt, dnf or opkg, so the update readers inherited from
LinuxDriver find no package manager, and QPKG updates are not implemented.
get_available_updates, refresh_available_updates and apply_updates are None,
the same opt-out as manage_service: netOrk's patch groups and compliance
report include a driver only when these are callable.

tests/test_qnap_qts.py

@@ -192,3 +192,13 @@ def test_services_cannot_be_controlled():
 
     assert callable(getattr(LinuxDriver, "manage_service", None))
     assert not callable(getattr(QnapQtsDriver, "manage_service", None))
+
+
+
+def test_updates_are_not_read_or_applied_here():
+    """QTS has no apt, dnf or opkg: the readers inherited from LinuxDriver find no
+    package manager, and QPKG updates are not implemented. Saying so keeps QNAP out
+    of netOrk's patch groups and compliance report, which ask whether these
+    methods are callable."""
+    for method in ("get_available_updates", "refresh_available_updates", "apply_updates"):
+        assert not callable(getattr(QnapQtsDriver, method, None)), method