tests: Mark adaptivedemux tests as sometimes timing out

... in addition to all the other issues that were ignored for them
already.

At least the reverse playback test regularly times out, waiting for a
download to finish that has already finished successfully.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1593>

subprojects/gst-integration-testsuites/testsuites/validate_known_issues.py

@@ -37,7 +37,11 @@ KNOWN_ISSUES = {
                 "level": "critical",
                 "summary": "The program stopped before some actions were executed",
                 "sometimes": True,
-            }
+            },
+            {
+                "timeout": True,
+                "sometimes": True,
+            },
         ]
     },
     "https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/issues/311": {