tests: on noble, expect default /etc/apt/sources.list

livecd-rootfs changes now place a default /etc/apt/sources.list
file in Ubuntu Noble and later where deb822 feature flag is enabled
in cloud-init.
Assert that the /etc/apt/sources.list content matches expectations.

tests/integration_tests/modules/test_apt_functionality.py

@@ -347,9 +347,11 @@ def test_disabled_apt_sources(self, class_client: IntegrationInstance):
             get_feature_flag_value(class_client, "APT_DEB822_SOURCE_LIST_FILE")
         )
         if feature_deb822:
-            assert class_client.execute(
-                f"test -f {ORIG_SOURCES_FILE}"
-            ).failed, f"Found unexpected {ORIG_SOURCES_FILE}"
+
+            assert (
+                cc_apt_configure.UBUNTU_DEFAULT_APT_SOURCES_LIST.strip()
+                == class_client.read_from_file(ORIG_SOURCES_FILE)
+            )
 
 
 DEFAULT_DATA_WITH_URI = _DEFAULT_DATA.format(