Fix missing source connections on merger

Change I453754052b75de3d4fac266dc73921c7ce9c075f had the effect that
no source connection was configured in the merger. This can be fixed
by changing the type check from issubclass to isinstance. Further also
the zuul and timer drivers don't need to be configured by the merger.

This also adds a test case for that.

Change-Id: If437123c3e8a0d63c5f852619e214cefa0892ce3
diff --git a/tests/base.py b/tests/base.py
index 0f8b3af..2bcd1ca 100755
--- a/tests/base.py
+++ b/tests/base.py
@@ -1928,7 +1928,7 @@
         self.sched.reconfigure(self.config)
         self.sched.resume()
 
-    def configure_connections(self):
+    def configure_connections(self, source_only=False):
         # Set up gerrit related fakes
         # Set a changes database so multiple FakeGerrit's can report back to
         # a virtual canonical database given by the configured hostname
@@ -1971,7 +1971,7 @@
 
         # Register connections from the config using fakes
         self.connections = zuul.lib.connections.ConnectionRegistry()
-        self.connections.configure(self.config)
+        self.connections.configure(self.config, source_only=source_only)
 
     def setup_config(self):
         # This creates the per-test configuration object.  It can be