image-diff: set bazelisk_test_targets to ""

Change-Id: I8e7c76d8b9485744142b50cdbe276fb91122810d

zuul.d/projects.yaml

diff --git a/zuul.d/projects.yaml b/zuul.d/projects.yaml
index 9c7584c..90477ef 100644
--- a/zuul.d/projects.yaml
+++ b/zuul.d/projects.yaml
@@ -177,6 +177,7 @@
     vars:
       bazelisk_artifacts:
         - "bazel-bin/plugins/{{ gerrit_plugin }}/{{ gerrit_plugin }}.js"
+      bazelisk_test_targets: ""
     templates:
       - non-core-plugin