artiq_influxdb: Support append() in dataset _Mock

This went undetected as append mods were not actually in use
in any part of the codebase previously.

artiq/frontend/artiq_influxdb.py

@@ -131,6 +131,9 @@ class _Mock:
     def __delitem__(self, k):
         pass
 
+    def append(self, v):
+        pass
+
 
 class Datasets:
     def __init__(self, filter_function, writer, init):