Turn off vm_tests for factory-2305.B branch

BUG=None
TEST=cbuildbot_config_unittests passes

Change-Id: I5350ca92538e42ef7460db5f8c2512fab16a7735
Reviewed-on: https://gerrit.chromium.org/gerrit/24089
Reviewed-by: Ryan Cui <rcui@chromium.org>
Reviewed-by: Eric Blake <eblake@chromium.org>
Tested-by: Eric Blake <eblake@chromium.org>
diff --git a/buildbot/cbuildbot_config.py b/buildbot/cbuildbot_config.py
index c81a892..48e910b 100755
--- a/buildbot/cbuildbot_config.py
+++ b/buildbot/cbuildbot_config.py
@@ -695,6 +695,7 @@
   master=True,
   push_overlays=constants.BOTH_OVERLAYS,
   boards=['lumpy'],
+  vm_tests=None,
 )
 
 internal_arm_paladin = internal_paladin.derive(arm)