test: add qubesdb mockup

Allow pylint checking calls into qubesdb too.

test-packages/qubesdb.py

@@ -0,0 +1,18 @@
+class QubesDB:
+    def read(self, key):
+        return b'testvm'
+
+    def write(self, key, value):
+        pass
+
+    def rm(self, key):
+        pass
+
+    def list(self, path):
+        return ['test']
+
+class Error(Exception):
+    pass
+
+class DisconnectedError(Error):
+    pass